netfilter: core: batch nf_unregister_net_hooks synchronize_net calls

re-add batching in nf_unregister_net_hooks().

Similar as before, just store an array with to-be-free'd rule arrays
on stack, then call synchronize_net once per batch.

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