Support vlan_group workaround implemented in XenServer kernels.

Support vlan_group workaround implemented in XenServer kernels.

Some Linux network drivers support a feature called "VLAN acceleration",
associated with a data structure called a "vlan_group".  A vlan_group is,
abstractly, a dictionary that maps from a VLAN ID (in the range 0...4095)
to a VLAN device, that is, a Linux network device associated with a
particular VLAN, e.g. "eth0.9" for VLAN 9 on eth0.

Some drivers that support VLAN acceleration have bugs that fall roughly
into the following categories:

    * Some NICs strip VLAN tags on receive if no vlan_group is registered,
      so that the tag is completely lost.

    * Some drivers size their receive buffers based on whether a vlan_group
      is enabled, meaning that a maximum size packet with a VLAN tag will
      not fit if a vlan_group is not configured.

    * On transmit some drivers expect that VLAN acceleration will be used
      if it is available (which can only be done if a vlan_group is
      configured).  In these cases, the driver may fail to parse the packet
      and correctly setup checksum offloading and/or TSO.

The correct long term solution is to fix these driver bugs.  To cope until
then, we have prepared a patch to the Linux kernel network stack that works
around these problems.  This commit adds support for the workaround
implemented by that patch.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>