af_unix: Annotate data-races around sk->sk_state in UNIX_DIAG.

While dumping AF_UNIX sockets via UNIX_DIAG, sk->sk_state is read
locklessly.

Let's use READ_ONCE() there.

Note that the result could be inconsistent if the socket is dumped
during the state change.  This is common for other SOCK_DIAG and
similar interfaces.

Fixes: c9da99e6 ("unix_diag: Fixup RQLEN extension report")
Fixes: 2aac7a2c ("unix_diag: Pending connections IDs NLA")
Fixes: 45a96b9b

 ("unix_diag: Dumping all sockets core")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>