rust,cred: update AlwaysRefCounted import to sync::aref

Update the import of `AlwaysRefCounted` in `cred.rs` to use `sync::aref`
instead of `types`.

This is part of the ongoing effort to move `ARef` and
`AlwaysRefCounted` to the `sync` module for better modularity.

Suggested-by: Benno Lossin <lossin@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1173


Signed-off-by: Shankari Anand <shankari.ak0208@gmail.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
[PM: subj tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>