[NETLINK]: Mirror UDP MSG_TRUNC semantics.

If the user passes MSG_TRUNC in via msg_flags, return
the full packet size not the truncated size.

Idea from Herbert Xu and Thomas Graf.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index c48b0f49..5890210 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1242,6 +1242,9 @@
 
 	scm_recv(sock, msg, siocb->scm, flags);
 
+	if (flags & MSG_TRUNC)
+		copied = skb->len;
+
 out:
 	netlink_rcv_wake(sk);
 	return err ? : copied;