[IPVS]: Use skb_forward_csum

As a path that forwards packets, IPVS should be using
skb_forward_csum instead of directly setting ip_summed
to CHECKSUM_NONE.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>