Unlike spa_async_thread that can get started only from spa_sync()
spa_async_thread_vd can get started from other contexts.
Additionally, spa_async_thread_vd does not really depend on
spa sync being enabled.
The incorrect assert could be triggered by importing a pool in the
read-only mode and then disconnecting one of its disks.
In this case spa_sync_on was false because the pool was read-only
and spa_async_thread_vd was started to handle SPA_ASYNC_REMOVE event.
Note: spa_async_thread_vd() currently exists only in FreeBSD, it was
split out of spa_async_thread() in r253990.
Discussed with: mav
MFC after: 2 weeks
13bacc7144