selftests: netfilter: remove unused cnt and simplify command testing

cnt was not used in nft_meta.sh
This patch also fixes 2 shellcheck SC2181 warnings:
"check exit code directly with e.g. 'if mycmd;', not indirectly with
$?."

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>