Linux 4: struct address_space no longer has backing_dev_info

Linux 4: struct address_space no longer has backing_dev_info

The backing_dev_info is only stored in the super_block now.

Reviewed-on: http://gerrit.openafs.org/11756
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 5cca05d1a1c9883e33c953b31c4cb32252474f77)

Change-Id: Id6f2f2dcc111c46ce1339595e145891758c46bab
Reviewed-on: http://gerrit.openafs.org/11761
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>