kselftests/bpf: use ping6 as the default ipv6 ping binary when it exists

At commit deee2cae27d1 ("kselftests/bpf: use ping6 as the default ipv6 ping
binary if it exists"), it fixed similar issues for shell script, but it
missed a same issue in the C code.

Fixes: 371e4fcc9d96 ("selftests/bpf: cgroup local storage-based network counters")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
CC: Philip Li <philip.li@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_netcnt.c

@@ -81,7 +81,10 @@ int main(int argc, char **argv)
 		goto err;
 	}
 
-	assert(system("ping localhost -6 -c 10000 -f -q > /dev/null") == 0);
+	if (system("which ping6 &>/dev/null") == 0)
+		assert(!system("ping6 localhost -c 10000 -f -q > /dev/null"));
+	else
+		assert(!system("ping -6 localhost -c 10000 -f -q > /dev/null"));
 
 	if (bpf_prog_query(cgroup_fd, BPF_CGROUP_INET_EGRESS, 0, NULL, NULL,
 			   &prog_cnt)) {