l2tp: correctly handle ancillary data in the ip6 recv path

l2tp_ip6 is incorrectly using the IPv4-specific ip_cmsg_recv to handle
ancillary data.  This means that socket options such as IPV6_RECVPKTINFO are
not honoured in userspace.

Convert l2tp_ip6 to use the IPv6-specific handler.

Ref: net/ipv6/udp.c

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Chris Elston <celston@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ip6.c

@@ -646,7 +646,7 @@ static int l2tp_ip6_recvmsg(struct kiocb *iocb, struct sock *sk,
 			    struct msghdr *msg, size_t len, int noblock,
 			    int flags, int *addr_len)
 {
-	struct inet_sock *inet = inet_sk(sk);
+	struct ipv6_pinfo *np = inet6_sk(sk);
 	struct sockaddr_l2tpip6 *lsa = (struct sockaddr_l2tpip6 *)msg->msg_name;
 	size_t copied = 0;
 	int err = -EOPNOTSUPP;
@@ -688,8 +688,8 @@ static int l2tp_ip6_recvmsg(struct kiocb *iocb, struct sock *sk,
 			lsa->l2tp_scope_id = IP6CB(skb)->iif;
 	}
 
-	if (inet->cmsg_flags)
-		ip_cmsg_recv(msg, skb);
+	if (np->rxopt.all)
+		ip6_datagram_recv_ctl(sk, msg, skb);
 
 	if (flags & MSG_TRUNC)
 		copied = skb->len;