scsi_debug: replace jiffy timers with hr timers

The driver supports two command delay interfaces, the original one whose
unit is a jiffy, and a newer one whose unit is a nanosecond. Each had
different implementations.  Keep both interfaces but simplify the
implemenation to use a single delay mechanism based on high resolution
timers.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>