rust: sync: add Arc::DATA_OFFSET

rust: sync: add Arc::DATA_OFFSET

This constant will be used to expose some offset constants from the Rust
Binder driver to tracepoints which are implemented in C. The constant is
usually equal to sizeof(refcount_t), but may be larger if T has a large
alignment.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://patch.msgid.link/20251203-binder-trace1-v1-1-22d3ffddb44e@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>