vlan: Keep NETIF_F_HW_CSUM similar to other software devices

vlan: Keep NETIF_F_HW_CSUM similar to other software devices

Vlan devices, like all other software devices, enable
NETIF_F_HW_CSUM feature.  However, unlike all the othe other
software devices, vlans will switch to using IP|IPV6_CSUM
features, if the underlying devices uses them.  In these situations,
checksum offload features on the vlan device can't be controlled
via ethtool.

This patch makes vlans keep HW_CSUM feature if the underlying
device supports checksum offloading.  This makes vlan devices
behave like other software devices, and restores control to the
user.

A side-effect is that some offload settings (typically UFO)
may be enabled on the vlan device while being disabled on the HW.
However, the GSO code will correctly process the packets. This
actually results in slightly better raw throughput.

Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>