netfilter: nf_tables: reduce trans->ctx.table references

nft_ctx is huge, it should not be stored in nft_trans at all,
most information is not needed.

Preparation patch to remove trans->ctx, no change in behaviour intended.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>