selftests: netfilter: add cpu counter check

run task on first CPU with netfilter counters reset and check
cpu meta after another ping

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>