LINUX: Switch to new bdi api for 4.12.

LINUX: Switch to new bdi api for 4.12.

super_setup_bdi() dynamically allocates backing_dev_info structures
for filesystems and cleans them up on superblock destruction.

Appears with Linux commit fca39346a55bb7196888ffc77d9e3557340d1d0b
Author: Jan Kara <jack@suse.cz>
Date:   Wed Apr 12 12:24:28 2017 +0200

Reviewed-on: https://gerrit.openafs.org/12614
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 63e530e7df0b8013bcc4421b0bba558d4f1d2d57)

Change-Id: I48a49ee8852bf842c24e7df0609fe2184bf45d90
Reviewed-on: https://gerrit.openafs.org/12626
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>