macvlan: Initialize vlan_features to turn on offload support.

Macvlan devices do not initialize vlan_features.  As a result,
any vlan devices configured on top of macvlans perform very poorly.
Initialize vlan_features based on the vlan features of the lower-level
device.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>