datapath: Fix Xen performance when adding a VLAN tag in presence of GSO.

datapath: Fix Xen performance when adding a VLAN tag in presence of GSO.

When the datapath needed to add a VLAN tag to a GSO packet, it would stick
it on the front of the packet and pass it along.  Then GSO would later fail
because it didn't know how to segment an 802.1Q packet.  So we need to
segment GSO packets by ourselves before we put the 802.1Q header on them.

We could avoid this problem if we could patch the main kernel, either to
add GSO support to the VLAN protocol or to support hardware-accelerated
VLAN tagging without VLAN groups configured).

Bug #1231.