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>

diff --cc net/netfilter/nf_tables_core.c
Simple merge

diff --cc net/netfilter/nft_payload.c
Simple merge