sr: implement ->free_disk to simplify refcounting

Simplify the refcounting and remove the need to clear disk->private_data
by implementing the ->free_disk method.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20220308055200.735835-9-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>