af_unix: dont send SCM_CREDENTIAL when dest socket is NULL

SCM_SCREDENTIALS should apply to write() syscalls only either source or destination
socket asserted SOCK_PASSCRED. The original implememtation in maybe_add_creds is wrong,
and breaks several LSB testcases ( i.e. /tset/LSB.os/netowkr/recvfrom/T.recvfrom).

Origionally-authored-by: Karel Srot <ksrot@redhat.com>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -1412,8 +1412,8 @@ static void maybe_add_creds(struct sk_buff *skb, const struct socket *sock,
 	if (UNIXCB(skb).cred)
 		return;
 	if (test_bit(SOCK_PASSCRED, &sock->flags) ||
-	    !other->sk_socket ||
-	    test_bit(SOCK_PASSCRED, &other->sk_socket->flags)) {
+	    (other->sk_socket &&
+	    test_bit(SOCK_PASSCRED, &other->sk_socket->flags))) {
 		UNIXCB(skb).pid  = get_pid(task_tgid(current));
 		UNIXCB(skb).cred = get_current_cred();
 	}