blk-mq: fix failure to decrement plug count on single rq removal

If we yank a 'same_queue_rq' request off the plug list, we should
also decrement the cached request count.

Fixes: 5f0ed774 ("block: sum requests in the plug structure")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -1946,8 +1946,10 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 		 */
 		if (list_empty(&plug->mq_list))
 			same_queue_rq = NULL;
-		if (same_queue_rq)
+		if (same_queue_rq) {
 			list_del_init(&same_queue_rq->queuelist);
+			plug->rq_count--;
+		}
 		list_add_tail(&rq->queuelist, &plug->mq_list);
 		plug->rq_count++;