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>

rust/kernel/sync/arc.rs

@@ -240,6 +240,9 @@ impl<T> Arc<T> {
         // `Arc` object.
         Ok(unsafe { Self::from_inner(inner) })
     }
+
+    /// The offset that the value is stored at.
+    pub const DATA_OFFSET: usize = core::mem::offset_of!(ArcInner<T>, data);
 }
 
 impl<T: ?Sized> Arc<T> {