scsi: scsi_ioctl: Consolidate the START STOP UNIT handling

Factor out a helper for the various flavors of START STOP UNIT command
ioctls.

Link: https://lore.kernel.org/r/20210724072033.1284840-22-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>