scsi: scsi_debug: Implement support for write protect

Teach scsi_debug to honor SWP in the Control Mode Page and report the
resulting WP state in the Device-Specific Parameter field.

In check_device_access_params() verify that commands that will write
the medium are permitted to do so.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>