ip6_gre: Do not allow segmentation offloads GRE_CSUM is enabled with FOU/GUE

This patch addresses the same issue we had for IPv4 where enabling GRE with
an inner checksum cannot be supported with FOU/GUE due to the fact that
they will jump past the GRE header at it is treated like a tunnel header.
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -1355,11 +1355,15 @@ static int ip6gre_newlink(struct net *src_net, struct net_device *dev,
 	dev->hw_features	|= GRE6_FEATURES;
 
 	if (!(nt->parms.o_flags & TUNNEL_SEQ)) {
-		/* TCP segmentation offload is not supported when we
-		 * generate output sequences.
+		/* TCP offload with GRE SEQ is not supported, nor
+		 * can we support 2 levels of outer headers requiring
+		 * an update.
 		 */
+		if (!(nt->parms.o_flags & TUNNEL_CSUM) ||
+		    (nt->encap.type == TUNNEL_ENCAP_NONE)) {
 			dev->features    |= NETIF_F_GSO_SOFTWARE;
 			dev->hw_features |= NETIF_F_GSO_SOFTWARE;
+		}
 
 		/* Can use a lockless transmit, unless we generate
 		 * output sequences