rust: kernel: use `core::ffi::CStr` method names

rust: kernel: use `core::ffi::CStr` method names

Prepare for `core::ffi::CStr` taking the place of `kernel::str::CStr` by
avoiding methods that only exist on the latter.

Also avoid `Deref<Target=BStr> for CStr` as that impl doesn't exist on
`core::ffi::CStr`.

Link: https://github.com/Rust-for-Linux/linux/issues/1075
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250704-core-cstr-prepare-v1-6-a91524037783@gmail.com
[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>