blk-mq: don't grab ->q_usage_counter in blk_mq_sched_bio_merge

blk_mq_sched_bio_merge is only called from blk-mq.c:blk_attempt_bio_merge(),
which is called when queue usage counter is grabbed already:

1) blk_mq_get_new_requests()

2) blk_mq_get_request()
- cached request in current plug owns one queue usage counter

So don't grab ->q_usage_counter in blk_mq_sched_bio_merge(), and more
importantly this nest way causes hang in blk_mq_freeze_queue_wait().

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211111085134.345235-2-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-sched.c

@@ -370,9 +370,6 @@ bool blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
 	bool ret = false;
 	enum hctx_type type;
 
-	if (bio_queue_enter(bio))
-		return false;
-
 	if (e && e->type->ops.bio_merge) {
 		ret = e->type->ops.bio_merge(q, bio, nr_segs);
 		goto out_put;
@@ -397,7 +394,6 @@ bool blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
 
 	spin_unlock(&ctx->lock);
 out_put:
-	blk_queue_exit(q);
 	return ret;
 }