netfilter: nf_tables: defer all object release via rcu

Now that all objects are released in the reverse order via the
transaction infrastructure, we can enqueue the release via
call_rcu to save one synchronize_rcu. For small rule-sets loaded
via nft -f, it now takes around 50ms less here.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>