netfilter: nf_tables: nft_payload: fix transport header base

We cannot use skb->transport_header since it's unset, use
pkt->xt.thoff instead.

Now possible using information made available through the x_tables
compatibility layer.
Reported-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_core.c

@@ -44,7 +44,7 @@ static bool nft_payload_fast_eval(const struct nft_expr *expr,
 	if (priv->base == NFT_PAYLOAD_NETWORK_HEADER)
 		ptr = skb_network_header(skb);
 	else
-		ptr = skb_transport_header(skb);
+		ptr = skb_network_header(skb) + pkt->xt.thoff;
 
 	ptr += priv->offset;
 

net/netfilter/nft_payload.c

@@ -36,7 +36,7 @@ static void nft_payload_eval(const struct nft_expr *expr,
 		offset = skb_network_offset(skb);
 		break;
 	case NFT_PAYLOAD_TRANSPORT_HEADER:
-		offset = skb_transport_offset(skb);
+		offset = pkt->xt.thoff;
 		break;
 	default:
 		BUG();