It doesn't matter. Because the reentry of quiesce&unquiesce queue is not
safe, must be avoided by other mechanism. otherwise, exceptions may
occur. Introduce mq_quiesce_lock looks saving possible synchronization
waits, but it should not happen. If really happen, we need fix it.
Sagi mentioned there may be nested queue quiesce, so I add .mq_quiesce_lock
to make this usage easy to support, meantime avoid percpu_ref warning
in such usage.
Anyway, not see any problem with adding .mq_quiesce_lock, so I'd suggest to
move on with this way.
I'm not sure there really are any nested queue quiesce paths, but if
there are, wouldn't we need to track the "depth" like how a queue freeze
works?
We might need it when the async quiesce is implemented for this,
because then quiesce will only "start" and in a different context wait
for the quiesced event (where we may need to add wakeup for waiters).
This is why I want to see this with the async quiesce piece.
