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

ping binary on some distros doesn't support "ping -6" anymore.

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>