xtables: Optimize nft_chain_zero_counters()

xtables: Optimize nft_chain_zero_counters()

If a chain name was given, make use of nftnl_chain_list_lookup_byname().
Streamline nft_chain_zero_rule_counters() to be suitable for calling
from nftnl_chain_list_foreach().

There is an unrelated optimization in here, too: Add batch job
NFT_COMPAT_CHAIN_ZERO only if it is a base chain. Since user-defined
chains don't have counters, there is no need to do anything for them.

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