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 <[e-mail]>
CC: Eric Dumazet <[e-mail]>
Signed-off-by: Pravin B Shelar <[e-mail]>
Acked-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/af_inet.c

···
 		       SKB_GSO_DODGY |
 		       SKB_GSO_TCP_ECN |
 		       SKB_GSO_GRE |
+		       SKB_GSO_TCPV6 |
 		       SKB_GSO_UDP_TUNNEL |
 		       0)))
 		goto out;

net/ipv4/gre.c

···
 	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;
 
···
 
 	/* 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;
···
 		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;