rust: list: undo unintended replacement of method name

rust: list: undo unintended replacement of method name

When we renamed `Opaque::raw_get` to `cast_into`, there was one
replacement that was not supposed to be there.

It does not cause an issue so far because it is inside a macro rule (the
`ListLinksSelfPtr` one) that is unused so far. However, it will start
to be used soon.

Thus fix it now.

Fixes: 64fb810bce03 ("rust: types: rename Opaque::raw_get to cast_into")
Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250719183649.596051-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>