net: l2tp: netlink: l2tp_nl_tunnel_send: set UDP6 checksum flags

This patch causes the proper attribute flags to be set,
in the case that IPv6 UDP checksums are disabled, so that
userspace ie. `ip l2tp show tunnel` knows about it.
Signed-off-by: Asbjoern Sloth Toennesen <asbjorn@asbjorn.st>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_netlink.c

@@ -384,6 +384,16 @@ static int l2tp_nl_tunnel_send(struct sk_buff *skb, u32 portid, u32 seq, int fla
 			if (nla_put_u8(skb, L2TP_ATTR_UDP_CSUM, !sk->sk_no_check_tx))
 				goto nla_put_failure;
 			break;
+#if IS_ENABLED(CONFIG_IPV6)
+		case AF_INET6:
+			if (udp_get_no_check6_tx(sk) &&
+			    nla_put_flag(skb, L2TP_ATTR_UDP_ZERO_CSUM6_TX))
+				goto nla_put_failure;
+			if (udp_get_no_check6_rx(sk) &&
+			    nla_put_flag(skb, L2TP_ATTR_UDP_ZERO_CSUM6_RX))
+				goto nla_put_failure;
+			break;
+#endif
 		}
 		if (nla_put_u16(skb, L2TP_ATTR_UDP_SPORT, ntohs(inet->inet_sport)) ||
 		    nla_put_u16(skb, L2TP_ATTR_UDP_DPORT, ntohs(inet->inet_dport)))