xfrm_decode_session_reverse
err = xfrm_decode_session_reverse(net, skb_in, flowi4_to_flowi(&fl4_dec), AF_INET);
err = xfrm_decode_session_reverse(net, skb, flowi6_to_flowi(&fl2), AF_INET6);
if (xfrm_decode_session_reverse(dev_net(skb->dev), newskb, fl1, family) < 0)