scsi: scsi_dh_alua: Warn if the first argument of alua_rtpg_queue() is NULL

Callers must provide a valid port group to alua_rtpg_queue().  Issue a
kernel warning if that is not the case.
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Tang Junhui <tang.junhui@zte.com.cn>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/device_handler/scsi_dh_alua.c

@@ -876,7 +876,7 @@ static bool alua_rtpg_queue(struct alua_port_group *pg,
 	unsigned long flags;
 	struct workqueue_struct *alua_wq = kaluad_wq;
 
-	if (!pg || scsi_device_get(sdev))
+	if (WARN_ON_ONCE(!pg) || scsi_device_get(sdev))
 		return false;
 
 	spin_lock_irqsave(&pg->lock, flags);