block: add missed trace_block_plug

After flush plug list, the list has no request, so we need to add a
trace_block_plug().

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Reviewed-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -1387,8 +1387,10 @@ get_rq:
 				if (__rq->q != q)
 					plug->should_sort = 1;
 			}
-			if (request_count >= BLK_MAX_REQUEST_COUNT)
+			if (request_count >= BLK_MAX_REQUEST_COUNT) {
 				blk_flush_plug_list(plug, false);
+				trace_block_plug(q);
+			}
 		}
 		list_add_tail(&req->queuelist, &plug->list);
 		drive_stat_acct(req, 1);