blk-mq: don't use plug->mq_list->q directly in blk_mq_run_dispatch_ops()

blk_mq_run_dispatch_ops() is defined as one macro, and plug->mq_list
will be changed when running 'dispatch_ops', so add one local variable
for holding request queue.
Reported-and-tested-by: Yi Zhang <yi.zhang@redhat.com>
Fixes: 4cafe86c ("blk-mq: run dispatch lock once in case of issuing from list")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -2521,7 +2521,9 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 	plug->rq_count = 0;
 
 	if (!plug->multiple_queues && !plug->has_elevator && !from_schedule) {
-		blk_mq_run_dispatch_ops(plug->mq_list->q,
+		struct request_queue *q = rq_list_peek(&plug->mq_list)->q;
+
+		blk_mq_run_dispatch_ops(q,
 				blk_mq_plug_issue_direct(plug, false));
 		if (rq_list_empty(plug->mq_list))
 			return;