[S390] dasd: fix reference counting in display method for proc/dasd/devices

Using the /proc/dasd/devices interface leaves the reference counter
of alias devices in an inconsistent state. A process that tries to set
such a device offline afterwards will hang.
The dasd_devices_show function returns immediately for alias devices
and this code path was missing a dasd_put_device call.

Signed-off-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dasd_proc.c

@@ -62,8 +62,10 @@ dasd_devices_show(struct seq_file *m, void *v)
 		return 0;
 	if (device->block)
 		block = device->block;
-	else
+	else {
+		dasd_put_device(device);
 		return 0;
+	}
 	/* Print device number. */
 	seq_printf(m, "%s", device->cdev->dev.bus_id);
 	/* Print discipline string. */