zfs: call spa_deadman on a taskqueue thread

[avg]

Summary

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().

[mav]

Part about taskqueue looks reasonable. Just shouldn't there be taskqueue_drain() calls near callout_drain()?

Part about SBT_MAX looks iffy to me. Not sure the cure is better then decease.

[avg]

In D9762#201594, @mav wrote:

Part about taskqueue looks reasonable. Just shouldn't there be taskqueue_drain() calls near callout_drain()?

Oops, I'll fix that.

Part about SBT_MAX looks iffy to me. Not sure the cure is better then decease.

There is a cleaner solution which is to add callout_init variant(s) that would accept func and arg, but I wanted to leave it for another day.

Thanks!

[avg]

drain the deadman task too

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I hope that my impatience will be forgiven if I commit this now.