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
without zeroing out the contents.  In particular there's no
guarantee that congested_fn is NULL, causing spurious oopses
when bdi_congested in the kernel tries to call it.

(adapted from commit 8e97cf6f215d5575c63d86eaec59031399f4beda)

Change-Id: I83755b6bb5ec2fada7e077c00d3d8edf8af1cae4
Reviewed-on: http://gerrit.openafs.org/6104
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>