Only the sr driver can handle SCSI passthrough requests, so move the call
to scsi_cmd_blk_ioctl() there.

Link: https://lore.kernel.org/r/20210724072033.1284840-9-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>