blk-mq: update comments on blk_mq_quiesce_queue()

Actually what we want to get from blk_mq_quiesce_queue()
isn't only to wait for completion of all ongoing .queue_rq().

In the typical context of canceling requests, we need to
make sure that the following is done in the dispatch path
before starting to cancel requests:

	- failed dispatched request is finished
	- busy dispatched request is requeued, and the STARTED
	flag is cleared

So update comment to keep code, doc and our expection consistent.
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -155,12 +155,13 @@ void blk_mq_unfreeze_queue(struct request_queue *q)
 EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);
 
 /**
- * blk_mq_quiesce_queue() - wait until all ongoing queue_rq calls have finished
+ * blk_mq_quiesce_queue() - wait until all ongoing dispatches have finished
  * @q: request queue.
  *
  * Note: this function does not prevent that the struct request end_io()
- * callback function is invoked. Additionally, it is not prevented that
- * new queue_rq() calls occur unless the queue has been stopped first.
+ * callback function is invoked. Once this function is returned, we make
+ * sure no dispatch can happen until the queue is unquiesced via
+ * blk_mq_unquiesce_queue().
  */
 void blk_mq_quiesce_queue(struct request_queue *q)
 {