net: Add GSO to vlan_features initialization

Just add GSO to vlan_features initialization, and update comments.

When we set offload features, vlan_dev_fix_features() will do more check.
In vlan_dev_fix_features(), final features is decided by
features of real device and vlan_features of real device.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>