block: Use pointer to backing_dev_info from request_queue

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>
Change-Id: I77fbb181de7e39c83fbfba8cfb128d6ace161f31
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git


Git-commit: 97419acd22a0bacc52dbc34d5bbc96d315e48acb
[riteshh@codeaurora.org: resolved merge conflicts]
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>