userquota_update_cache failure

[behlendorf]

Reproducible on aarch64 when creating a new pool.

SPL: Loaded module v0.7.0-rc1_8_g0d26756 (DEBUG mode)
SPLAT: Loaded module v0.7.0-rc1_8_g0d26756 (DEBUG mode)
zavl: module license 'CDDL' taints kernel. 
Disabling lock debugging due to kernel taint
ZFS: Loaded module v0.7.0-rc1_134_g33f4ca2 (DEBUG mode), ZFS pool version 5000, ZFS filesystem version 5
SPL: using hostid 0x00000000
VERIFY(-1 <= diff && diff <= 1) failed
PANIC at avl.c:268:avl_find()
Showing stack for process 25275
CPU: 1 PID: 25275 Comm: txg_sync Tainted: P           O 3.14.79-84 #1
Call trace:
[<ffffffc001088e40>] dump_backtrace+0x0/0x128
[<ffffffc001088f8c>] show_stack+0x24/0x30
[<ffffffc0018835ac>] dump_stack+0x88/0xac
[<ffffffbffc0cd6dc>] $x+0x44/0x60 [spl]
[<ffffffbffc0cd7a0>] $x+0xa8/0xe0 [spl]
[<ffffffbffc14d1d4>] avl_find+0xa4/0x148 [zavl]
[<ffffffbffc26114c>] userquota_update_cache+0x44/0x100 [zfs]
[<ffffffbffc263190>] do_userobjquota_update.part.2+0x60/0xa0 [zfs]
[<ffffffbffc263e0c>] dmu_objset_do_userquota_updates+0x30c/0x6d0 [zfs]
[<ffffffbffc28f04c>] $x+0x174/0x608 [zfs]
[<ffffffbffc2b4504>] $x+0x3ec/0x10a8 [zfs]
[<ffffffbffc2c9490>] txg_sync_thread+0x308/0x610 [zfs]
[<ffffffbffc0c8c3c>] thread_generic_wrapper+0x84/0xd8 [spl]
[<ffffffc0010d4150>] kthread+0xd8/0xf0