scsi_debug: fix prevent_allow+verify regressions

Ruediger Meier observed a regression with the PREVENT ALLOW MEDIUM
REMOVAL command in lk 3.19:

	http://www.spinics.net/lists/util-linux-ng/msg11448.html



Inspection indicated the same regression with VERIFY(10).

The patch is against lk 3.19.3 and also works with lk 4.3.0 .  With this
patch both commands are accepted and do nothing.

ChangeLog:

 - fix the lk 3.19 regression so that the PREVENT ALLOW MEDIUM REMOVAL
   command is supported once again

 - same fix for VERIFY(10)

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