nft: CMD_ZERO needs a rule cache

nft: CMD_ZERO needs a rule cache

In order to zero rule counters, they have to be fetched from kernel. Fix
this for both standalone calls as well as xtables-restore --noflush.

Fixes: b5cb6e631c828 ("nft-cache: Fetch only chains in nft_chain_list_get()")
Fixes: 09cb517949e69 ("xtables-restore: Improve performance of --noflush operation")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>