[PATCH RFC v2 0/3] Support fraglist GRO/GSO

[Steffen Klassert <steffen.klassert@secunet.com>]

This patchset adds support to do GRO/GSO by chaining packets
of the same flow at the SKB frag_list pointer. This avoids
the overhead to merge payloads into one big packet, and
on the other end, if GSO is needed it avoids the overhead
of splitting the big packet back to the native form.

Patch 1 Enables UDP GRO by default.
Patch 2 adds the core infrastructure to do fraglist
GRO/GSO.
Patch 3 enables UDP to use fraglist GRO/GSO if no GRO
supported socket is found.

I have only forwarding performance measurements so far:

I used used my IPsec forwarding test setup for this:

           ------------         ------------
        -->| router 1 |-------->| router 2 |--
        |  ------------         ------------  |
        |                                     |
        |       --------------------          |
        --------|Spirent Testcenter|<----------
                --------------------

net-next (December 27th):

Single stream UDP frame size 1460 Bytes: 1.385.000 fps (16.2 Gbps).

----------------------------------------------------------------------

net-next (December 27th) + hack to enable forwarding for standard UDP GRO:

Single stream UDP frame size 1460 Bytes: 1.759.000 fps (20.5 Gbps).

----------------------------------------------------------------------

net-next (December 27th) + fraglist UDP GRO/GSO:

Single stream UDP frame size 1460 Bytes: 3.016.000 fps (35.2 Gbps).

-----------------------------------------------------------------------

Known issues:

- With this patchset fraglist GRO/GSO is done always on forwarding.
  This will change as soon as we agreed on how to decide beteen
  standard and fraglist GRO/GSO.
  
Changes from v1:

- Add IPv6 support.
- Split patchset to enable UDP GRO by default before adding
  fraglist GRO support.
- Mark fraglist GRO packets as CHECKSUM_NONE.
- Take a refcount on the first segment skb when doing fraglist
  segmentation. With this we can use the same error handling
  path as with standard segmentation.