Device removal panics on 32-bit systems

The issue is caused by an incorrect usage of the sizeof() operator
in vdev_obsolete_sm_object(): on 64-bit systems this is not an issue
since both "uint64_t" and "uint64_t*" are 8 bytes in size. However on
32-bit systems pointers are 4 bytes long which is not supported by
zap_lookup_impl(). Trying to remove a top-level vdev on a 32-bit system
will cause the following failure:

VERIFY3(0 == vdev_obsolete_sm_object(vd, &obsolete_sm_object)) failed (0 == 22)
PANIC at vdev_indirect.c:833:vdev_indirect_sync_obsolete()
Showing stack for process 1315
CPU: 6 PID: 1315 Comm: txg_sync Tainted: P           OE   4.4.69+ #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-2.fc27 04/01/2014
 c1abc6e7 0ae10898 00000286 d4ac3bc0 c14397bc da4cd7d8 d4ac3bf0 d4ac3bd0
 d790e7ce d7911cc1 00000523 d4ac3d00 d790e7d7 d7911ce4 da4cd7d8 00000341
 da4ce664 da4cd8c0 da33fa6e 49524556 28335946 3d3d2030 65647620 626f5f76
Call Trace:
 [<>] dump_stack+0x58/0x7c
 [<>] spl_dumpstack+0x23/0x27 [spl]
 [<>] spl_panic.cold.0+0x5/0x41 [spl]
 [<>] ? dbuf_rele+0x3e/0x90 [zfs]
 [<>] ? zap_lookup_norm+0xbe/0xe0 [zfs]
 [<>] ? zap_lookup+0x57/0x70 [zfs]
 [<>] ? vdev_obsolete_sm_object+0x102/0x12b [zfs]
 [<>] vdev_indirect_sync_obsolete+0x3e1/0x64d [zfs]
 [<>] ? txg_verify+0x1d/0x160 [zfs]
 [<>] ? dmu_tx_create_dd+0x80/0xc0 [zfs]
 [<>] vdev_sync+0xbf/0x550 [zfs]
 [<>] ? mutex_lock+0x10/0x30
 [<>] ? txg_list_remove+0x9f/0x1a0 [zfs]
 [<>] ? zap_contains+0x4d/0x70 [zfs]
 [<>] spa_sync+0x9f1/0x1b10 [zfs]
 ...
 [<>] ? kthread_stop+0x110/0x110

This commit simply corrects the "integer_size" parameter used to lookup
the vdev's ZAP object.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes openzfs#8790

module/zfs/vdev_indirect.c

@@ -15,6 +15,7 @@
 
 /*
  * Copyright (c) 2014, 2017 by Delphix. All rights reserved.
+ * Copyright (c) 2019, loli10K <ezomori.nozomu@gmail.com>. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -902,7 +903,7 @@ vdev_obsolete_sm_object(vdev_t *vd, uint64_t *sm_obj)
 	}
 
 	int error = zap_lookup(vd->vdev_spa->spa_meta_objset, vd->vdev_top_zap,
-	    VDEV_TOP_ZAP_INDIRECT_OBSOLETE_SM, sizeof (sm_obj), 1, sm_obj);
+	    VDEV_TOP_ZAP_INDIRECT_OBSOLETE_SM, sizeof (uint64_t), 1, sm_obj);
 	if (error == ENOENT) {
 		*sm_obj = 0;
 		error = 0;