Merge branch 'virtio_net_ufo'

Vladislav Yasevich says:

====================
Restore UFO support to virtio_net devices

commit 3d0ad09412ffe00c9afa201d01effdb6023d09b4
Author: Ben Hutchings <ben@decadent.org.uk>
Date:   Thu Oct 30 18:27:12 2014 +0000

    drivers/net: Disable UFO through virtio

Turned off UFO support to virtio-net based devices due to issues
with IPv6 fragment id generation for UFO packets.  The issue
was that IPv6 UFO/GSO implementation expects the fragment id
to be supplied in skb_shinfo().  However, for packets generated
by the VMs, the fragment id is not supplied which causes all
IPv6 fragments to have the id of 0.

The problem is that turning off UFO support on tap/macvtap
as well as virtio devices caused issues with migrations.
Migrations would fail when moving a vm from a kernel supporting
expecting UFO to work to the newer kernels that disabled UFO.

This series provides a partial solution to address the migration
issue.  The series allows us to track whether skb_shinfo()->ip6_frag_id
has been set by treating value of 0 as unset.
This lets GSO code to generate fragment ids if they are necessary
(ex: packet was generated by VM or packet socket).

Since v3:
  - Resolved build issue when IPv6 is a module.
  - Removed trailing white space.

Since v2:
  - Rebase and rebuild to make sure everything works.  No changes
    to the patches were done.

Since v1:
  - Removed the skb bit and use value of 0 as tracker.
  - Used Eric's suggestion to set fragment id as 0x80000000 if id
    generation procedure yeilded a 0 result.
  - Consolidated ipv6 id genration code.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>