netfilter: nf_tables: prepare nft_object for lookups via hashtable

Add a 'key' structure for object, so we can look them up by name + table
combination (the name can be the same in each table).

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