netfilter: ip6t_NPT: Use csum_partial()

[ Some fixes went into mainstream before this patch, so I needed
  to rebase it upon the current tree, that's why it's different from
  the original one posted on the list --pablo ]

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>