rust: sync: implement == operator for ARef

rust: sync: implement == operator for ARef

Rust Binder wants to perform a comparison between ARef<Task> and &Task,
so define the == operator for ARef<_> when compared with another ARef<_>
or just a reference. The operator is implemented in terms of the same
operator applied to the inner type.

Note that PartialEq<U> cannot be implemented because it would overlap
with the impl for ARef<U>.

Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260324-close-fd-check-current-v3-1-b94274bedac7@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>