datapath: Add module parameter to allow TSO with vlans.

datapath: Add module parameter to allow TSO with vlans.

We currently perform GSO on packets before adding a vlan tag,
which is reliable but hurts performance.  Even NICs that support
TSO on vlan tagged packets typically expect vlan acceleration to
be used.  Before 2.6.37 we can't use vlan acceleration and must
place the tag in the packet itself, which is risky when used with
TSO.  However, if the driver is known to work with internally
tagged packets and TSO this exposes a module parameter to enable it.

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