gre: Fix GREv4 TCPv6 segmentation.

For ipv6 traffic, GRE can generate packet with strange GSO
bits, e.g. ipv4 packet with SKB_GSO_TCPV6 flag set.  Therefore
following patch relaxes check in inet gso handler to allow
such packet for segmentation.
This patch also fixes wrong skb->protocol set that was done in
gre_gso_segment() handler.

Reported-by: Steinar H. Gunderson <sesse@google.com>
CC: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/af_inet.c

@@ -1293,6 +1293,7 @@ static struct sk_buff *inet_gso_segment(struct sk_buff *skb,
 		       SKB_GSO_DODGY |
 		       SKB_GSO_TCP_ECN |
 		       SKB_GSO_GRE |
+		       SKB_GSO_TCPV6 |
 		       SKB_GSO_UDP_TUNNEL |
 		       0)))
 		goto out;

net/ipv4/gre.c

@@ -121,6 +121,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 	int ghl = GRE_HEADER_SECTION;
 	struct gre_base_hdr *greh;
 	int mac_len = skb->mac_len;
+	__be16 protocol = skb->protocol;
 	int tnl_hlen;
 	bool csum;
 
@@ -150,7 +151,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 
 	/* setup inner skb. */
 	if (greh->protocol == htons(ETH_P_TEB)) {
-		struct ethhdr *eth = eth_hdr(skb);
+		struct ethhdr *eth = (struct ethhdr *)skb_inner_mac_header(skb);
 		skb->protocol = eth->h_proto;
 	} else {
 		skb->protocol = greh->protocol;
@@ -199,6 +200,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 		skb_reset_mac_header(skb);
 		skb_set_network_header(skb, mac_len);
 		skb->mac_len = mac_len;
+		skb->protocol = protocol;
 	} while ((skb = skb->next));
 out:
 	return segs;