Revert "block: Merge bio before checking ->cached_rq"

This reverts commit 23f3e32.

blk-mq sched bio merge still needs request to grab queue usage counter,
so we can't simply call blk_mq_attempt_bio_merge() when queue usage
counter isn't held.

Fixes: 23f3e32 ("block: Merge bio before checking ->cached_rq")
Cc: Xiao Ni <[email protected]>
Reported-by: Yi Zhang <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Ming Lei

block/blk-mq.c

@@ -2878,16 +2878,15 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
 
 	if (!plug)
 		return NULL;
+	rq = rq_list_peek(&plug->cached_rq);
+	if (!rq || rq->q != q)
+		return NULL;
 
 	if (blk_mq_attempt_bio_merge(q, *bio, nsegs)) {
 		*bio = NULL;
 		return NULL;
 	}
 
-	rq = rq_list_peek(&plug->cached_rq);
-	if (!rq || rq->q != q)
-		return NULL;
-
 	type = blk_mq_get_hctx_type((*bio)->bi_opf);
 	hctx_type = rq->mq_hctx->type;
 	if (type != hctx_type &&