blk-mq: fix for trace_block_plug()

The trace point is for tracing plug event of each request
queue instead of each task, so we should check the request
count in the plug list from current queue instead of
current task.
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Reviewed-by: NJeff Moyer <jmoyer@redhat.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -1380,7 +1380,7 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio)
 	plug = current->plug;
 	if (plug) {
 		blk_mq_bio_to_request(rq, bio);
-		if (list_empty(&plug->mq_list))
+		if (!request_count)
 			trace_block_plug(q);
 		else if (request_count >= BLK_MAX_REQUEST_COUNT) {
 			blk_flush_plug_list(plug, false);