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

tools/testing/selftests/netfilter/nft_meta.sh

@@ -90,7 +90,7 @@ check_one_counter()
 	if [ $? -ne 0 ];then
 		echo "FAIL: $cname, want \"$want\", got"
 		ret=1
-		ip netns exec "$ns0" nft list counter inet filter $counter
+		ip netns exec "$ns0" nft list counter inet filter $cname
 	fi
 }