veth: allow to setup multicast address for veth device

We can only setup multicast address for network device when
net_device_ops->ndo_set_rx_mode is not null.

Some configurations need to add multicast address for net
device, such as netfilter cluster match module.

Add a fake ndo_set_rx_mode function to allow this operation.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>