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>