block: Get rid of blk_get_backing_dev_info()

blk_get_backing_dev_info() is now a simple dereference. Remove that
function and simplify some code around that.

Change-Id: I8ed9ffc88b53f3967adf93c7bc66be4282500e6d
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
Git-commit: efa7c9f97e3ef624e9a398bf69c15f58eea9f0e8
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git


[riteshh@codeaurora.org: Resolved merge conflicts]
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>