vlan: convert VLAN devices to use ndo_fix_features()

Note: get_flags was actually broken, because it should return the
flags capped with vlan_features. This is now done implicitly by
limiting netdev->hw_features.

RX checksumming offload control is (and was) broken, as there was no way
before to say whether it's done for tagged packets.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>