dm rq: leverage blk_mq_queue_busy() to check for outstanding IO

Now that request-based dm-multipath only supports blk-mq, make use of
the newly introduced blk_mq_queue_busy() to check for outstanding IO --
rather than (ab)using the block core's in_flight counters.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/dm-rq.c

@@ -130,11 +130,11 @@ static void rq_end_stats(struct mapped_device *md, struct request *orig)
  */
 static void rq_completed(struct mapped_device *md, int rw, bool run_queue)
 {
-	atomic_dec(&md->pending[rw]);
-
 	/* nudge anyone waiting on suspend queue */
-	if (!md_in_flight(md))
+	if (unlikely(waitqueue_active(&md->wait))) {
+		if (!blk_mq_queue_busy(md->queue))
 			wake_up(&md->wait);
+	}
 
 	/*
 	 * dm_put() must be at the end of this function. See the comment above
@@ -436,7 +436,6 @@ ssize_t dm_attr_rq_based_seq_io_merge_deadline_store(struct mapped_device *md,
 static void dm_start_request(struct mapped_device *md, struct request *orig)
 {
 	blk_mq_start_request(orig);
-	atomic_inc(&md->pending[rq_data_dir(orig)]);
 
 	if (unlikely(dm_stats_used(&md->stats))) {
 		struct dm_rq_target_io *tio = tio_from_request(orig);