datapath: Use vlan acceleration for vlan operations.

datapath: Use vlan acceleration for vlan operations.

Using the kernel vlan acceleration has a number of benefits:
it enables hardware tagging, allows usage of TSO and checksum
offloading, and is generally easier to manipulate.  This switches
the vlan actions to use skb->vlan_tci field for any necessary
changes.  In places that do not support vlan acceleration in a way
that we can use (in particular kernels before 2.6.37) we perform
any necessary conversions, such as tagging and GSO before the
packet leaves Open vSwitch.

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