Quiet lockdep circular lock warning

This is a false positive.  The full lockdep warning is below.  The
jist of it is:
    the existing dependency chain (in reverse order) is:
        -> #3 (&dp->dp_lock){+.+.}:
    dsl_pool_dirty_space+0x70/0x2d0 [zfs]
    dbuf_dirty+0x778/0x31d0 [zfs]

        -> #2 (&db->db_mtx){+.+.}:
    dmu_buf_will_dirty+0x6b/0x6c0 [zfs]
    bpobj_iterate_impl+0xbe6/0x1410 [zfs]
    spa_sync+0x1756/0x29b0 [zfs]
    txg_sync_thread+0x6d6/0x1370 [zfs]

        -> #1 (&bpo->bpo_lock){+.+.}:
    bpobj_space+0x63/0x470 [zfs]
    dsl_scan_active+0x340/0x3d0 [zfs]
    txg_sync_thread+0x3f2/0x1370 [zfs]

        -> #0 (&tx->tx_sync_lock){+.+.}:
    __mutex_lock+0xef/0x1380
    txg_kick+0x61/0x420 [zfs]
    dsl_pool_need_dirty_delay+0x1c7/0x3f0 [zfs]

Different instances of txg_sync_thread manage different dsl_pools,
which contain different bpo_lock instances.  Stacks #1 and #2
therefore can't be referring to the same bpo_lock instance, so this
can't be an actual loop.

The patch adds an ID field (bpo_id) to bpo_lock which is unique to each
instance.  This bpo_id is passed as the class to the
mutex_enter_nested call in bpobj_space.  This effectively makes this
locking call invisible to lockdep.

[  125.297679] ======================================================
[  125.301361] hardirqs last  enabled at (28216159): [<ffffffff885cef12>] handle_mm_fault+0x582/0x7d0
[  125.302863] WARNING: possible circular locking dependency detected
[  125.302865] 4.19.29-4.19.0-debug-99d9c44b25c08f51 #1 Tainted: G           O
[  125.309056] hardirqs last disabled at (28216160): [<ffffffff88530f22>] get_page_from_freelist+0x3b2/0x31a0
[  125.318009] ------------------------------------------------------
[  125.318011] mdtagent/13103 is trying to acquire lock:
[  125.324203] softirqs last  enabled at (28215144): [<ffffffff89c00631>] __do_softirq+0x631/0x868
[  125.331591] 00000000679a2c2b (
[  125.341249] softirqs last disabled at (28215139): [<ffffffff8813ef80>] irq_exit+0x150/0x170
[  125.347425] &tx->tx_sync_lock){+.+.}, at: txg_kick+0x61/0x420 [zfs]
[  125.378920]
               but task is already holding lock:
[  125.384762] 00000000ec7e955e (&dp->dp_lock){+.+.}, at: dsl_pool_need_dirty_delay+0x8b/0x3f0 [zfs]
[  125.393679]
               which lock already depends on the new lock.

[  125.401861]
               the existing dependency chain (in reverse order) is:
[  125.409347]
               -> #3 (&dp->dp_lock){+.+.}:
[  125.414710]        dsl_pool_dirty_space+0x70/0x2d0 [zfs]
[  125.420068]        dbuf_dirty+0x778/0x31d0 [zfs]
[  125.424723]        dmu_write_uio_dnode+0xfe/0x300 [zfs]
[  125.429990]        dmu_write_uio_dbuf+0xa0/0x100 [zfs]
[  125.435174]        zfs_write+0x18bd/0x1f70 [zfs]
[  125.439837]        zpl_write_common_iovec+0x15e/0x380 [zfs]
[  125.445452]        zpl_iter_write+0x1c6/0x2a0 [zfs]
[  125.450334]        __vfs_write+0x3a2/0x5b0
[  125.454440]        vfs_write+0x15d/0x460
[  125.458365]        ksys_write+0xb1/0x170
[  125.462295]        do_syscall_64+0x9b/0x400
[  125.466491]        entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  125.472069]
               -> #2 (&db->db_mtx){+.+.}:
[  125.477337]        dmu_buf_will_dirty+0x6b/0x6c0 [zfs]
[  125.482509]        bpobj_iterate_impl+0xbe6/0x1410 [zfs]
[  125.487863]        spa_sync+0x1756/0x29b0 [zfs]
[  125.492442]        txg_sync_thread+0x6d6/0x1370 [zfs]
[  125.497499]        thread_generic_wrapper+0x1ff/0x2a0 [spl]
[  125.503080]        kthread+0x2e7/0x3e0
[  125.506839]        ret_from_fork+0x24/0x50
[  125.510937]
               -> #1 (&bpo->bpo_lock){+.+.}:
[  125.516464]        bpobj_space+0x63/0x470 [zfs]
[  125.521038]        dsl_scan_active+0x340/0x3d0 [zfs]
[  125.526049]        txg_sync_thread+0x3f2/0x1370 [zfs]
[  125.531106]        thread_generic_wrapper+0x1ff/0x2a0 [spl]
[  125.536684]        kthread+0x2e7/0x3e0
[  125.540439]        ret_from_fork+0x24/0x50
[  125.544535]
               -> #0 (&tx->tx_sync_lock){+.+.}:
[  125.550295]        __mutex_lock+0xef/0x1380
[  125.554529]        txg_kick+0x61/0x420 [zfs]
[  125.558841]        dsl_pool_need_dirty_delay+0x1c7/0x3f0 [zfs]
[  125.564715]        dmu_tx_assign+0x13b/0xe40 [zfs]
[  125.569557]        zfs_write+0xb5e/0x1f70 [zfs]
[  125.574130]        zpl_write_common_iovec+0x15e/0x380 [zfs]
[  125.579746]        zpl_iter_write+0x1c6/0x2a0 [zfs]
[  125.584628]        __vfs_write+0x3a2/0x5b0
[  125.588726]        vfs_write+0x15d/0x460
[  125.592654]        ksys_write+0xb1/0x170
[  125.596580]        do_int80_syscall_32+0xf1/0x460
[  125.601287]        entry_INT80_compat+0x8a/0xa0
[  125.605817]
               other info that might help us debug this:

[  125.613825] Chain exists of:
                 &tx->tx_sync_lock --> &db->db_mtx --> &dp->dp_lock

[  125.624174]  Possible unsafe locking scenario:

[  125.630099]        CPU0                    CPU1
[  125.634633]        ----                    ----
[  125.639166]   lock(&dp->dp_lock);
[  125.642485]                                lock(&db->db_mtx);
[  125.648232]                                lock(&dp->dp_lock);
[  125.654071]   lock(&tx->tx_sync_lock);
[  125.657823]
                *** DEADLOCK ***

[  125.663743] 3 locks held by mdtagent/13103:
[  125.667928]  #0: 0000000016b80a9c (&f->f_pos_lock){+.+.}, at: __fdget_pos+0xb1/0xe0
[  125.675591]  #1: 00000000d0227133 (sb_writers#12){.+.+}, at: vfs_write+0x332/0x460
[  125.683167]  #2: 00000000ec7e955e (&dp->dp_lock){+.+.}, at: dsl_pool_need_dirty_delay+0x8b/0x3f0 [zfs]
[  125.692517]
               stack backtrace:
[  125.696878] CPU: 28 PID: 13103 Comm: mdtagent Tainted: G           O      4.19.29-4.19.0-debug-99d9c44b25c08f51 #1
[  125.707226] Hardware name: Ciara Technologies 1x16-X7p-L-team/X7P-MB, BIOS P2.12 03/05/2019
[  125.715578] Call Trace:
[  125.718038]  dump_stack+0x91/0xeb
[  125.721364]  print_circular_bug.isra.16+0x30b/0x5b0
[  125.726249]  ? save_trace+0xd6/0x240
[  125.729827]  __lock_acquire+0x41be/0x4f10
[  125.733841]  ? debug_show_all_locks+0x2d0/0x2d0
[  125.738373]  ? debug_show_all_locks+0x2d0/0x2d0
[  125.742907]  ? kernel_text_address+0x6d/0x100
[  125.747276]  ? __save_stack_trace+0x73/0xd0
[  125.751468]  ? quarantine_put+0xb0/0x150
[  125.755396]  ? lock_acquire+0x153/0x330
[  125.759242]  lock_acquire+0x153/0x330
[  125.762966]  ? txg_kick+0x61/0x420 [zfs]
[  125.766896]  ? lock_contended+0xd60/0xd60
[  125.770958]  ? txg_kick+0x61/0x420 [zfs]
[  125.774885]  __mutex_lock+0xef/0x1380
[  125.778603]  ? txg_kick+0x61/0x420 [zfs]
[  125.782572]  ? txg_kick+0x61/0x420 [zfs]
[  125.786500]  ? __mutex_add_waiter+0x160/0x160
[  125.790866]  ? sched_clock+0x5/0x10
[  125.794359]  ? __mutex_add_waiter+0x160/0x160
[  125.798721]  ? __cv_destroy+0x9e/0x180 [spl]
[  125.803002]  ? cv_destroy_wakeup+0xc0/0xc0 [spl]
[  125.807628]  ? __mutex_unlock_slowpath+0xf3/0x660
[  125.812384]  ? zio_wait+0x419/0x660 [zfs]
[  125.816442]  ? txg_kick+0x61/0x420 [zfs]
[  125.820412]  txg_kick+0x61/0x420 [zfs]
[  125.824209]  dsl_pool_need_dirty_delay+0x1c7/0x3f0 [zfs]
[  125.829563]  dmu_tx_assign+0x13b/0xe40 [zfs]
[  125.833872]  ? dmu_tx_count_write+0x42f/0x700 [zfs]
[  125.838800]  zfs_write+0xb5e/0x1f70 [zfs]
[  125.842816]  ? sched_clock+0x5/0x10
[  125.846358]  ? zfs_close+0x1e0/0x1e0 [zfs]
[  125.850457]  ? sched_clock+0x5/0x10
[  125.853951]  ? sched_clock_cpu+0x18/0x170
[  125.857971]  ? check_flags.part.23+0x480/0x480
[  125.862418]  ? __lock_acquire+0xe3b/0x4f10
[  125.866526]  ? __fget+0x2d6/0x3e0
[  125.869852]  ? sched_clock+0x5/0x10
[  125.873345]  ? sched_clock_cpu+0x18/0x170
[  125.877402]  zpl_write_common_iovec+0x15e/0x380 [zfs]
[  125.882453]  ? __lock_acquire+0xe3b/0x4f10
[  125.886596]  ? zpl_read_common_iovec+0x2b0/0x2b0 [zfs]
[  125.891779]  zpl_iter_write+0x1c6/0x2a0 [zfs]
[  125.896139]  __vfs_write+0x3a2/0x5b0
[  125.899717]  ? __fdget_pos+0xb1/0xe0
[  125.903297]  ? kernel_read+0x130/0x130
[  125.907049]  ? __mutex_add_waiter+0x160/0x160
[  125.911407]  ? lock_acquire+0x153/0x330
[  125.915248]  ? lock_acquire+0x153/0x330
[  125.919089]  ? rcu_read_lock_sched_held+0xeb/0x120
[  125.923887]  ? rcu_sync_lockdep_assert+0x76/0xb0
[  125.928509]  ? __sb_start_write+0xf0/0x260
[  125.932614]  vfs_write+0x15d/0x460
[  125.936022]  ksys_write+0xb1/0x170
[  125.939425]  ? __ia32_sys_read+0xb0/0xb0
[  125.943353]  ? poll_select_set_timeout+0xc0/0xc0
[  125.947974]  ? trace_hardirqs_on_thunk+0x1a/0x1c
[  125.952601]  ? do_int80_syscall_32+0x17/0x460
[  125.956960]  do_int80_syscall_32+0xf1/0x460
[  125.961145]  entry_INT80_compat+0x8a/0xa0

include/sys/bpobj.h

@@ -55,6 +55,7 @@ typedef struct bpobj_phys {
 
 typedef struct bpobj {
 	kmutex_t	bpo_lock;
+        int		bpo_id;
 	objset_t	*bpo_os;
 	uint64_t	bpo_object;
 	int		bpo_epb;

module/zfs/bpobj.c

@@ -143,6 +143,8 @@ bpobj_free(objset_t *os, uint64_t obj, dmu_tx_t *tx)
 	VERIFY3U(0, ==, dmu_object_free(os, obj, tx));
 }
 
+static atomic_t next_bpo_id = ATOMIC_INIT(0);
+
 int
 bpobj_open(bpobj_t *bpo, objset_t *os, uint64_t object)
 {
@@ -172,6 +174,7 @@ bpobj_open(bpobj_t *bpo, objset_t *os, uint64_t object)
 	bpo->bpo_havecomp = (doi.doi_bonus_size > BPOBJ_SIZE_V0);
 	bpo->bpo_havesubobj = (doi.doi_bonus_size > BPOBJ_SIZE_V1);
 	bpo->bpo_phys = bpo->bpo_dbuf->db_data;
+	bpo->bpo_id = atomic_inc_return(&next_bpo_id);
 	return (0);
 }
 
@@ -818,7 +821,7 @@ int
 bpobj_space(bpobj_t *bpo, uint64_t *usedp, uint64_t *compp, uint64_t *uncompp)
 {
 	ASSERT(bpobj_is_open(bpo));
-	mutex_enter(&bpo->bpo_lock);
+	mutex_enter_nested(&bpo->bpo_lock, bpo->bpo_id);
 
 	*usedp = bpo->bpo_phys->bpo_bytes;
 	if (bpo->bpo_havecomp) {