af_unix: Annotate data-race of sk->sk_state in unix_accept().

Once sk->sk_state is changed to TCP_LISTEN, it never changes.

unix_accept() takes the advantage and reads sk->sk_state without
holding unix_state_lock().

Let's use READ_ONCE() there.

Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>