PF_GS_CHKSUM
{ .type = PF_GS_CHKSUM, .off = _OUT(pf_chksum), .arg_u32 = PF_MD5_DIGEST_LENGTH, .cb = snl_attr_get_bytes },
nlattr_add(nw, PF_GS_CHKSUM, PF_MD5_DIGEST_LENGTH, V_pf_status.pf_chksum);