Linux: 2.6.32 - Adapt to writeback changes

Linux: 2.6.32 - Adapt to writeback changes

Adapt to the writeback changes in kernel 2.6.32
- Since we define our own backing_dev, it needs to be registered with
the writeback code and attached to the super_block.  Otherwise it
might get ignored when writeback is needed.
- Each backing_dev now gets its own kernel thread.  The name of the
thread is based on the registered name - the openafs one will appear
as "flush-afs".

Reviewed-on: http://gerrit.openafs.org/574
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>