ipv6: gre: support SIT encapsulation

gre_gso_segment() chokes if SIT frames were aggregated by GRO engine.

Fixes: 61c1db7f ("ipv6: sit: add GSO/TSO support")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/gre_offload.c

@@ -36,7 +36,8 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 				  SKB_GSO_TCP_ECN |
 				  SKB_GSO_GRE |
 				  SKB_GSO_GRE_CSUM |
-				  SKB_GSO_IPIP)))
+				  SKB_GSO_IPIP |
+				  SKB_GSO_SIT)))
 		goto out;
 
 	if (!skb->encapsulation)