scsi: scsi_debug: ARRAY_SIZE and FF_MEDIA_IO

Reviewer suggested using the ARRAY_SIZE macro. That reduced one of the subtle
inter-dependencies in the parser's tables.

It is important that commands which simulate media access, indicate this in the
flags for that command. The flag to do that was FF_DIRECT_IO. On reflection
FF_MEDIA_IO seems a more accurate description.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>