scsi: scsi_debug: Fix cmd duration calculation

In some cases, sdebug_defer::cmpl_ts (completion timestamp) wasn't being
properly set when REQ_HIPRI was given. Fix that and improve code to only
call ktime_get_boottime_ns() for commands with REQ_HIPRI set as cmpl_ts is
only used in that case.

Link: https://lore.kernel.org/r/20210304014107.307625-1-dgilbert@interlog.com


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