vlan: turn on unicast filtering on vlan device

Currently vlan device inherits unicast filtering flag from underlying
device. If underlying device doesn't support unicast filter, this will
put vlan device into promiscuous mode when it's stacked.

Tun on IFF_UNICAST_FLT on the vlan device in any case so that it does
not go into promiscuous mode needlessly. If underlying device does not
support unicast filtering, that device will enter promiscuous mode.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>