writeback: update comment in inode_io_list_move_locked

The @head can be wb->b_dirty_time, so update the comment.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Wang Long <wanglong19@meituan.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>