npf_addr_cksum
cksum = npf_addr_cksum(cksum, enpc.npc_alen, enpc.npc_ips[which], addr);
uint16_t npf_addr_cksum(uint16_t, int, const npf_addr_t *,
ip->ip_sum = npf_addr_cksum(ipsum, alen, oaddr, addr);
cksum = npf_addr_cksum(*ocksum, alen, oaddr, addr);