Cast __wsum from/to __sum16 is wrong. Instead, apply appropriate
conversion function: csum_unfold() or csum_fold().
[ The original patch has been modified to undo the final ~ that
csum_fold returns. We only need to fold the 32-bit word that
results from the checksum calculation into a 16-bit to ensure
that the original subnet is restored appropriately. Spotted by
Ulrich Weber. ]
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>