scsi: scsi_ioctl: Factor SG_IO handling into a helper

Split the SG_IO handler from the main scsi_ioctl() routine.

Link: https://lore.kernel.org/r/20210724072033.1284840-24-hch@lst.de


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