[S390] cio: Fix ungroup vs. machine check race in ccwgroup.

There are two ways for a ccwgroup device to be unregistered:
Via the ungroup attribute, or when on of the slave devices is killed
by a machine check. As we have to unregister the ccwgroup device
via a callback, we'll now have to check whether it hasn't been
already unregistered because of a machine check.
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/ccwgroup.c

@@ -75,8 +75,10 @@ static void ccwgroup_ungroup_callback(struct device *dev)
 	struct ccwgroup_device *gdev = to_ccwgroupdev(dev);
 
 	mutex_lock(&gdev->reg_mutex);
-	__ccwgroup_remove_symlinks(gdev);
-	device_unregister(dev);
+	if (device_is_registered(&gdev->dev)) {
+		__ccwgroup_remove_symlinks(gdev);
+		device_unregister(dev);
+	}
 	mutex_unlock(&gdev->reg_mutex);
 }