As Paolo pointed out, the result of ping IPv6 address depends on
the running distro. So explicitly checking the available ping feature,
as e.g. do the bareudp.sh self-tests.

Fixes: 8b3170e0 ("selftests: net: using ping6 for IPv6 in udpgro_fwd.sh")
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Link: https://lore.kernel.org/r/825ee22b-4245-dbf7-d2f7-a230770d6e21@163.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>