rust: transmute: simplify code with Rust 1.80.0 `split_at_*checked()`

rust: transmute: simplify code with Rust 1.80.0 `split_at_*checked()`

`feature(split_at_checked)` [1] has been stabilized in Rust 1.80.0 [2],
which is older than our new minimum Rust version (Rust 1.85.0).

Thus simplify the code using `split_at_*checked()`.

Link: https://github.com/rust-lang/rust/issues/119128
Link: https://github.com/rust-lang/rust/pull/124678
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-14-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>