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

At commit deee2cae ("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: 371e4fcc

 ("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>