Revert commit 8bd274934987 ("block: fix bdi vs gendisk lifetime mismatch")

Revert commit 8bd274934987 ("block: fix bdi vs gendisk lifetime mismatch")

Orabug: 28968102

8bd274934987 adds a new element at the end of struct backing_dev_info.
struct backing_dev_info is embedded inside struct request_queue
and is usually allocated during request_queue allocation. However, some
out of tree modules also allocate this struct separately using
sizeof() or via direct inclusion in their own structs. This patch
essentially breaks KABI for such modules and can cause a mismatch in the
size of the allocated struct and lead to kernel crashes.

Signed-off-by: Ashish Samant <ashish.samant@oracle.com>
Reviewed-by: Jianchao Wang <jianchao.w.wang@oracle.com>
Reviewed-by: Shan Hai <shan.hai@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>