l2tp: fix info leak in l2tp_ip6_recvmsg()

The L2TP code for IPv6 fails to initialize the l2tp_conn_id member of
struct sockaddr_l2tpip6 and therefore leaks four bytes kernel stack
in l2tp_ip6_recvmsg() in case msg_name is set.

Initialize l2tp_conn_id with 0 to avoid the info leak.
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ip6.c

@@ -690,6 +690,7 @@ static int l2tp_ip6_recvmsg(struct kiocb *iocb, struct sock *sk,
 		lsa->l2tp_addr = ipv6_hdr(skb)->saddr;
 		lsa->l2tp_flowinfo = 0;
 		lsa->l2tp_scope_id = 0;
+		lsa->l2tp_conn_id = 0;
 		if (ipv6_addr_type(&lsa->l2tp_addr) & IPV6_ADDR_LINKLOCAL)
 			lsa->l2tp_scope_id = IP6CB(skb)->iif;
 	}