nd6_prelist_add
if ((error = nd6_prelist_add(&pr0, NULL, &pr)) != 0)
if ((error = nd6_prelist_add(&pr0, NULL, NULL)) != 0)
int nd6_prelist_add(struct nd_prefix *, struct nd_defrouter *,
error = nd6_prelist_add(new, dr, &newpr);