block: Dynamically allocate and refcount backing_dev_info

Instead of storing backing_dev_info inside struct request_queue,
allocate it dynamically, reference count it, and free it when the last
reference is dropped. Currently only request_queue holds the reference
but in the following patch we add other users referencing
backing_dev_info.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
Change-Id: Ibcee7b4c014018f9243cd3edbfd9c4a8877c3862
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git


Git-commit: d03f6cdc1fc422accb734c7c07a661a0018d8631
[riteshh@codeaurora.org: resolved merge conflicts]
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>