l2tp: fix info leak via getsockname()

The L2TP code for IPv6 fails to initialize the l2tp_unused member of
struct sockaddr_l2tpip6 and that for leaks two bytes kernel stack via
the getsockname() syscall. Initialize l2tp_unused with 0 to avoid the
info leak.
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ip6.c

@@ -410,6 +410,7 @@ static int l2tp_ip6_getname(struct socket *sock, struct sockaddr *uaddr,
 	lsa->l2tp_family = AF_INET6;
 	lsa->l2tp_flowinfo = 0;
 	lsa->l2tp_scope_id = 0;
+	lsa->l2tp_unused = 0;
 	if (peer) {
 		if (!lsk->peer_conn_id)
 			return -ENOTCONN;