ethtool: Add support for vlan accleration.

Now that vlan acceleration is handled consistently regardless of usage,
it is possible to enable and disable it at will.  This adds support for
Ethtool operations that change the offloading status for debugging
purposes, similar to other forms of hardware acceleration.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>