rust: revocable: implement Revocable::access()

Implement an unsafe direct accessor for the data stored within the
Revocable.

This is useful for cases where we can prove that the data stored within
the Revocable is not and cannot be revoked for the duration of the
lifetime of the returned reference.

Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Link: https://lore.kernel.org/r/20250428140137.468709-2-dakr@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>