rust: binder: enable `clippy::cast_lossless`

Before Rust 1.29.0, Clippy introduced the `cast_lossless` lint [1]:

> Rust's `as` keyword will perform many kinds of conversions, including
> silently lossy conversions. Conversion functions such as `i32::from`
> will only perform lossless conversions. Using the conversion functions
> prevents conversions from becoming silently lossy if the input types
> ever change, and makes it clear for people reading the code that the
> conversion is lossless.

While this does not eliminate unchecked `as` conversions, it makes such
conversions easier to scrutinize. It also has the slight benefit of
removing a degree of freedom on which to bikeshed. Thus apply the
changes and enable the lint in the Binder Rust driver -- no functional
change intended.

Link: https://rust-lang.github.io/rust-clippy/master/index.html#cast_lossless [1]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Assisted-by: Codex:gpt-5
Signed-off-by: Tamir Duberstein <tamird@kernel.org>
Link: https://patch.msgid.link/20260526-binder-strict-provenance-v2-5-a41d89c29bc5@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>