dm: don't schedule delayed run of the queue if nothing to do

In request-based DM's dm_request_fn(), if blk_peek_request() returns
NULL just return.  Avoids unnecessary blk_delay_queue().
Reported-by: NJens Axboe <axboe@fb.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1963,7 +1963,7 @@ static void dm_request_fn(struct request_queue *q)
 	while (!blk_queue_stopped(q)) {
 		rq = blk_peek_request(q);
 		if (!rq)
-			goto delay_and_out;
+			goto out;
 
 		/* always use block 0 to find the target for flushes for now */
 		pos = 0;