selftests: forwarding: Exit with error when missing interfaces

Returning 0 gives a false sense of success when the required modules did
not even manage to be initialized and register the required net devices.

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