sd: implement ->free_disk to simplify refcounting

Implement the ->free_disk method to to put struct scsi_disk when the last
gendisk reference count goes away.  This removes the need to clear
->private_data and thus freeze the queue on unbind.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20220308055200.735835-8-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>