PF_ST_PROTO
snl_add_msg_attr_u16(&nw, PF_ST_PROTO, filter->proto);
nlattr_add_u8(nw, PF_ST_PROTO, s->key[PF_SK_WIRE]->proto);
{ .type = PF_ST_PROTO, .off = _OUT(proto), .cb = nlattr_get_uint16 },