[SCSI] scsi_debug: invalidate protection info for unmapped region

When UNMAP command is issued with the data integrity support enabled,
the protection info for the unmapped region is remain unchanged.
So READ command for the region later on causes data integrity failure.

This fixes it by invalidating protection info for the unmapped region
by filling with 0xff pattern.

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