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.

Change-Id: Iec92b14c5a1d4f25dd6e3abe9c18c121b2ae3fa4
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
Git-commit: d03f6cdc1fc422accb734c7c07a661a0018d8631
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git


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