inet_nat64_mask
d->u32[3] = inet_nat64_mask(s->u32[0], p->u32[3],
d->u32[0] = inet_nat64_mask(s->u32[3], p->u32[0], pfxlen);
d->u32[3] = inet_nat64_mask(s->u32[0], p->u32[0], pfxlen);
d->u32[0] = inet_nat64_mask(s->u32[3], p->u32[3],