netfilter: ip6t_NPT: fix IPv6 NTP checksum calculation

csum16_add() has a broken carry detection, should be:
sum += sum < (__force u16)b;

Instead of fixing csum16_add, remove the custom checksum
functions and use the generic csum_add/csum_sub ones.

Signed-off-by: Ulrich Weber <ulrich.weber@sophos.com>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>