LU-9558 llite: port lustre to unified handling of bdi

LU-9558 llite: port lustre to unified handling of bdi

For the linux 4.12 kernel the bdi handling was unified for all
file systems. This was done by allocating struct backing_dev_info
separately instead of embedding it inside superblock. For older
kernels we move all the bdi handling lustre does to the function
super_setup_bdi_name() which is what exist in the latest kernels.

Linux-commit: 9594caf216dc0fe3e318b34af0127276db661241

Change-Id: I5af60ea3661e3d3a97973fd99a79c28dcd1ce1cc
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/28511
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>