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>