[S390] device_schedule_callback() for dcssblk.

Unregistering a device from within a device attribute handler leads to
a deadlock. Need to use device_schedule_callback() to unregister device
in error path.
Signed-off-by: Gerald Schaefer <geraldsc@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dcssblk.c

@@ -193,6 +193,12 @@ dcssblk_segment_warn(int rc, char* seg_name)
 	}
 }
 
+static void dcssblk_unregister_callback(struct device *dev)
+{
+	device_unregister(dev);
+	put_device(dev);
+}
+
 /*
  * device attribute for switching shared/nonshared (exclusive)
  * operation (show + store)
@@ -276,8 +282,7 @@ dcssblk_shared_store(struct device *dev, struct device_attribute *attr, const ch
 	blk_cleanup_queue(dev_info->dcssblk_queue);
 	dev_info->gd->queue = NULL;
 	put_disk(dev_info->gd);
-	device_unregister(dev);
-	put_device(dev);
+	rc = device_schedule_callback(dev, dcssblk_unregister_callback);
 out:
 	up_write(&dcssblk_devices_sem);
 	return rc;