Linux 3.13: Check return value from bdi_init

The use of the bdi_init function now gets a warning because the
return value is unused and the function is now defined with
the warn_unused_result attribute.

Assign and check the return value.

Change-Id: I78ae4ea356aef8c9dabe75179ad67db1fd64c28b
Reviewed-on: http://gerrit.openafs.org/10530
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>