scsi: sr: Add error handling support for add_disk()

We never checked for errors on add_disk() as this function returned
void. Now that this is fixed, use the shiny new error handling.

Just put the cdrom kref and have the unwinding be done by
sr_kref_release().

Link: https://lore.kernel.org/r/20211015233028.2167651-3-mcgrof@kernel.org


Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>