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

unix_stream_read_skb() is called from sk->sk_data_ready() context
where unix_state_lock() is not held.

Let's use READ_ONCE() there.

Fixes: 77462de1

 ("af_unix: Add read_sock for stream socket types")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>