netfilter: nf_tables: move bind list_head into relevant subtypes

Only nft_trans_chain and nft_trans_set subtypes use the
trans->binding_list member.

Add a new common binding subtype and move the member there.

This reduces size of all other subtypes by 16 bytes on 64bit platforms.

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