nbd: quiesce request queues to make sure no submissions are inflight

Unlike blk_mq_stop_hw_queues, blk_mq_quiesce_queue respects the
submission path rcu grace. quiesce the queue before iterating
on live tags.

Reviewed-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>