in6p_cksum
in6p->in6p_cksum = optval;
optval = in6p->in6p_cksum;
if (in6p->in6p_cksum != -1) {
in6p->in6p_cksum != -1) {
off = in6p->in6p_cksum;
inp->in6p_cksum = -1;
inp->in6p_cksum = -1; /* just to be sure */