zfs: call spa_deadman on a taskqueue thread

zfs: call spa_deadman on a taskqueue thread

callout(9) prohibits callout functions from sleeping.
illumos mutexes are emulated using sx(9).
spa_deadman() calls vdev_deadman() and the latter acquires vq_lock.

As a result we can get a more confusing panic instead of a specific
panic or no panic:
sleepq_add: td 0xfffff80019669960 to sleep on wchan 0xfffff8001cff4d88 with sleeping prohibited

This change adds another level of indirection where the deadman
callout schedules spa_deadman() to be executed on taskqueue_thread.

While there, use callout_schedule(0 instead of callout_reset()
in spa_sync().

Discussed with:	mav
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D9762