selftests: net: using ping6 for IPv6 in udpgro_fwd.sh

udpgro_fwd.sh output following message:
  ping: 2001:db8:1::100: Address family for hostname not supported

Using ping6 when pinging IPv6 addresses.

Fixes: a062260a

 ("selftests: net: add UDP GRO forwarding self-tests")
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>