[SCSI] scsi_debug: fix false positive logical block reference tag check fail

Reading partially unwritten sectors generates a false positive logical
block reference tag check failure when DIF is enabled.

This bug is caused by missing ei_lba increment in loop of dif_verify()
when unwritten sector is skipped.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>