af_unix: Annotate data-race of sk->sk_shutdown in sk_diag_fill().

While dumping sockets via UNIX_DIAG, we do not hold unix_state_lock().

Let's use READ_ONCE() to read sk->sk_shutdown.

Fixes: e4e541a8

 ("sock-diag: Report shutdown for inet and unix sockets (v2)")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>