ipvs: fix CHECKSUM_PARTIAL for TUN method

 	The recent change in IP_VS_XMIT_TUNNEL to set
CHECKSUM_NONE is not correct. After adding IPIP header
skb->csum becomes invalid but the CHECKSUM_PARTIAL
case must be supported. So, use skb_forward_csum() which is
most suitable for us to allow local clients to send IPIP
to remote real server.
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>

net/netfilter/ipvs/ip_vs_xmit.c

@@ -222,7 +222,7 @@ ip_vs_dst_reset(struct ip_vs_dest *dest)
 		__ret = ip_vs_confirm_conntrack(skb, cp);	\
 	if (__ret == NF_ACCEPT) {				\
 		nf_reset(skb);					\
-		(skb)->ip_summed = CHECKSUM_NONE;		\
+		skb_forward_csum(skb);				\
 	}							\
 	__ret;							\
 })