rust: types: require `ForeignOwnable::into_foreign` return non-null

The intended implementations of `ForeignOwnable` will not return null
pointers from `into_foreign`, as this would render the implementation of
`try_from_foreign` useless. Current users of `ForeignOwnable` rely on
`into_foreign` returning non-null pointers. So require `into_foreign` to
return non-null pointers.

Suggested-by: Benno Lossin <lossin@kernel.org>
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250612-pointed-to-v3-2-b009006d86a1@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>