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 <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_tables_core.c

···
 	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

···
 		offset = skb_network_offset(skb);
 		break;
 	case NFT_PAYLOAD_TRANSPORT_HEADER:
-		offset = skb_transport_offset(skb);
+		offset = pkt->xt.thoff;
 		break;
 	default:
 		BUG();