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

As small optimisation, unix_stream_connect() prefetches the client's
sk->sk_state without unix_state_lock() and checks if it's TCP_CLOSE.

Later, sk->sk_state is checked again under unix_state_lock().

Let's use READ_ONCE() for the first check and TCP_CLOSE directly for
the second check.

Fixes: 1da177e4

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