rust: specify when `ARef` is thread safe

rust: specify when `ARef` is thread safe

An `ARef` behaves just like the `Arc` when it comes to thread safety, so
we can reuse the thread safety comments from `Arc` here.

This is necessary because without this change, the Rust compiler will
assume that things are not thread safe even though they are.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230531145939.3714886-4-aliceryhl@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>