MFuser/delphij/zfs-arc-rebase@r281754:

In r256613, taskqueue_enqueue_locked() have been modified to release the
task queue lock before returning.  In r276665, taskqueue_drain_all() will
call taskqueue_enqueue_locked() to insert the barrier task into the queue,
but did not reacquire the lock after it but later code expects the lock
still being held (e.g. TQ_SLEEP()).

The barrier task is special and if we release then reacquire the lock,
there would be a small race window where a high priority task could sneak
into the queue.  Looking more closely, the race seems to be tolerable but
is undesirable from semantics standpoint.

To solve this, in taskqueue_drain_tq_queue(), instead of directly calling
taskqueue_enqueue_locked(), insert the barrier task directly without
releasing the lock.

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