netfilter: nf_tables: add and use nft_thoff helper

[ Upstream commit 2d7b4ace ]

This allows to change storage placement later on without changing readers.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: 28427f36 ("netfilter: nft_exthdr: Fix non-linear header modification")
Signed-off-by: Sasha Levin <sashal@kernel.org>