selftests: forwarding: Use PING6 instead of ping for ipv6 multipath test

On Debian jessie ping can not handle IPv6 addresses so the command
fails. Use PING6 which is set to ping6.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>