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>