unix_*_recvmsg() returns zero-length sockaddr if the sender is an
unnamed AF_UNIX socket. Change it to return a two-byte sockaddr with
just the address family, to be consistent with unix_getname().

Signed-off-by: Pavel Pergamenshchik <ppergame@gmail.com>

---
Minimal example at http://xzrq.net/uaddrwtf.c
Solaris/OS X print 16 and 16. Linux prints 0 and 2 as described above.

--- a/net/unix/af_unix.c	2010-04-01 16:02:33.000000000 -0700
+++ b/net/unix/af_unix.c	2010-04-21 20:17:43.564703748 -0700
@@ -1634,9 +1634,13 @@
 static void unix_copy_addr(struct msghdr *msg, struct sock *sk)
 {
 	struct unix_sock *u = unix_sk(sk);
+	struct sockaddr_un *sunaddr;

-	msg->msg_namelen = 0;
-	if (u->addr) {
+	if (!u->addr) {
+		msg->msg_namelen = sizeof(short);
+		sunaddr = msg->msg_name;
+		sunaddr->sun_family = AF_UNIX;
+	} else {
 		msg->msg_namelen = u->addr->len;
 		memcpy(msg->msg_name, u->addr->name, u->addr->len);
 	}
--
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