scsi: scsi_debug: reset injection flags for every_nth > 0

If every_nth > 0, the injection flags must be reset for commands that
aren't supposed to fail (i.e. that aren't "nth"). Otherwise, commands
will continue to fail, like in the every_nth < 0 case.

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