vlan: Add features for stacked vlan device

vlan: Add features for stacked vlan device

Stacked vlan devices curretly have few features (GRO, HIGHDMA, LLTX).
Since we have software fallbacks in case the NIC can not handle some
features for multiple vlans, we can add the same features as the lower
vlan devices for stacked vlan devices.

This allows stacked vlan devices to create large (GSO) packets and not to
segment packets. Those packets will be segmented by software on the real
device, or even can be segmented by the NIC once TSO for multiple vlans
becomes enabled by the following patches.

The exception is those related to FCoE, which does not have a software
fallback.

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>