blk-mq: Exit queue on alloc failure

Fixes usage counter when a request could not be allocated.
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -258,8 +258,10 @@ struct request *blk_mq_alloc_request(struct request_queue *q, int rw, gfp_t gfp,
 		ctx = alloc_data.ctx;
 	}
 	blk_mq_put_ctx(ctx);
-	if (!rq)
+	if (!rq) {
+		blk_mq_queue_exit(q);
 		return ERR_PTR(-EWOULDBLOCK);
+	}
 	return rq;
 }
 EXPORT_SYMBOL(blk_mq_alloc_request);