selftests: netfilter: fix nft_meta.sh error reporting

selftests: netfilter: fix nft_meta.sh error reporting

When some test directly done with check_one_counter() fails,
counter variable is undefined. This patch calls ip with cname
which avoids errors like:
FAIL: oskuidcounter, want "packets 2", got
Error: syntax error, unexpected newline, expecting string
list counter inet filter
                        ^
Error is now correctly rendered:
FAIL: oskuidcounter, want "packets 2", got
table inet filter {
	counter oskuidcounter {
		packets 1 bytes 84
	}
}

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