IP_GRE: Fix kernel panic in IP_GRE with GRE csum.

Due to IP_GRE GSO support, GRE can recieve non linear skb which
results in panic in case of GRE_CSUM.  Following patch fixes it by
using correct csum API.

Bug introduced in commit 6b78f16e (gre: add GSO support)
Signed-off-by: NPravin B Shelar <pshelar@nicira.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -963,8 +963,12 @@ static netdev_tx_t ipgre_tunnel_xmit(struct sk_buff *skb, struct net_device *dev
 			ptr--;
 		}
 		if (tunnel->parms.o_flags&GRE_CSUM) {
+			int offset = skb_transport_offset(skb);
+
 			*ptr = 0;
-			*(__sum16 *)ptr = ip_compute_csum((void *)(iph+1), skb->len - sizeof(struct iphdr));
+			*(__sum16 *)ptr = csum_fold(skb_checksum(skb, offset,
+								 skb->len - offset,
+								 0));
 		}
 	}