blk-mq: mq plug list breakage

We switched to plug mq_list for mq, but some code are still using old list.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 2dc8de8..88d4e86 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -950,7 +950,7 @@
 
 		if (plug) {
 			blk_mq_bio_to_request(rq, bio);
-			if (list_empty(&plug->list))
+			if (list_empty(&plug->mq_list))
 				trace_block_plug(q);
 			else if (request_count >= BLK_MAX_REQUEST_COUNT) {
 				blk_flush_plug_list(plug, false);