s390/ccwgroup: obtain extra reference for asynchronous processing

Commit 0b60f9ea
"s390: use device_remove_file_self() instead of device_schedule_callback()"
changed ccwgroup to use an extra work queue instead of
device_schedule_callback. This function obtained an extra device
reference for its async work which is missing in the new implementation
and results in a "freeing memory with a lock still held" BUG. Fix
this by obtaining an extra reference for the async work.
Reported-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/ccwgroup.c

@@ -227,6 +227,7 @@ static void ccwgroup_ungroup_workfn(struct work_struct *work)
 		container_of(work, struct ccwgroup_device, ungroup_work);
 
 	ccwgroup_ungroup(gdev);
+	put_device(&gdev->dev);
 }
 
 static void ccwgroup_release(struct device *dev)
@@ -412,8 +413,10 @@ static int ccwgroup_notifier(struct notifier_block *nb, unsigned long action,
 {
 	struct ccwgroup_device *gdev = to_ccwgroupdev(data);
 
-	if (action == BUS_NOTIFY_UNBIND_DRIVER)
+	if (action == BUS_NOTIFY_UNBIND_DRIVER) {
+		get_device(&gdev->dev);
 		schedule_work(&gdev->ungroup_work);
+	}
 
 	return NOTIFY_OK;
 }