block: Use pointer to backing_dev_info from request_queue

block: Use pointer to backing_dev_info from request_queue

BugLink: http://bugs.launchpad.net/bugs/1659111
We will want to have struct backing_dev_info allocated separately from
struct request_queue. As the first step add pointer to backing_dev_info
to request_queue and convert all users touching it. No functional
changes in this patch.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
(back ported from commit dc3b17cc8bf21307c7e076e7c778d5db756f7871)

Conflicts:
	block/blk-cgroup.c
	block/blk-core.c
	block/blk-settings.c
	block/blk-sysfs.c
	block/blk-wbt.c
	drivers/block/aoe/aoeblk.c
	drivers/block/drbd/drbd_nl.c
	drivers/md/dm.c
	drivers/md/md.c

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>