selftests: forwarding: ping{6, }_do(): Allow passing ping arguments

Make the ping routine more generic by allowing passing arbitrary ping
command-line arguments.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>