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 <[e-mail]>
Cc: James Chapman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/l2tp/l2tp_ip6.c

···
 	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;