af_unix: Don't pass struct socket to maybe_add_creds().

We will move SOCK_PASS{CRED,PIDFD,SEC} from struct socket.flags
to struct sock for better handling with SOCK_PASSRIGHTS.

Then, we don't need to access struct socket in maybe_add_creds().

Let's pass struct sock to maybe_add_creds() and its caller
queue_oob().

While at it, we append the unix_ prefix and fix double spaces
around the pid assignment.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>