netfilter: nf_tables: avoid usage of embedded nft_ctx

nft_ctx is stored in nft_trans object, but nft_ctx is large
(48 bytes on 64-bit platforms), it should not be embedded in
the transaction structures.

Reduce its usage so we can remove it eventually.

This replaces trans->ctx.chain with the chain pointer
already available in nft_trans_chain structure.

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