Linux: make sure backing_dev_info is zeroed

Linux: make sure backing_dev_info is zeroed

The afs backing_dev_info structure is allocated dynamically with
kmalloc, which doesn't zero out the contents.  In particular
there's no guarantee that congested_fn is NULL, causing spurious
oopses when bdi_congested tries to call it.

Change-Id: Ieeed870905627a132a0d1bd2f15cf80443b36303
Reviewed-on: http://gerrit.openafs.org/5334
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>