udp_cksum
VNET_DEFINE(int, udp_cksum) = 1;
&VNET_NAME(udp_cksum), 0, "compute udp checksum");
VNET_DECLARE(int, udp_cksum);
#define V_udp_cksum VNET(udp_cksum)
outudp->uh_sum = udp_cksum(&Src, &Dst, outpacket, datalen);
u_int16_t udp_cksum(struct sockaddr_in6 *, struct sockaddr_in6 *,