writeback: correct comment for move_expired_inodes()

The function scans @delaying_queue and stops at the first inode
whose dirtied_when is after *work->older_than_this. So the expired
ones being moved are those before *work->older_than_this. Correct
the comment here.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index be3efc4..fd255c0 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -248,7 +248,7 @@
 }
 
 /*
- * Move expired (dirtied after work->older_than_this) dirty inodes from
+ * Move expired (dirtied before work->older_than_this) dirty inodes from
  * @delaying_queue to @dispatch_queue.
  */
 static int move_expired_inodes(struct list_head *delaying_queue,