netfilter: nf_tables: store chain pointer in rule transaction

Currently the chain can be derived from trans->ctx.chain, but
the ctx will go away soon.

Thus add the chain pointer to nft_trans_rule structure itself.

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