block: ensure cached plug request matches the current queue

If we're driving multiple devices, we could have pre-populated the cache
for a different device. Ensure that the empty request matches the current
queue.

Fixes: 47c122e3 ("block: pre-allocate requests if plug is started and is a batch")
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -2540,7 +2540,7 @@ static inline struct request *blk_mq_get_request(struct request_queue *q,
 		struct request *rq;
 
 		rq = rq_list_peek(&plug->cached_rq);
-		if (rq) {
+		if (rq && rq->q == q) {
 			if (unlikely(!submit_bio_checks(bio)))
 				return NULL;
 			if (blk_attempt_bio_merge(q, bio, nsegs, same_queue_rq))