nft-cache: Support partial cache per table

nft-cache: Support partial cache per table

Accept a builtin_table pointer in __nft_build_cache() and pass it along
when fetching chains and rules to operate on that table only (unless the
pointer is NULL).

Make use of it in nft_chain_list_get() since that accepts a table name
and performs a builtin table lookup internally already.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>