UBI: fix resource de-allocation

GregKH asked to fix UBI which has fake device release method. Indeed,
we have to free UBI device description object from the release method,
because otherwise we'll oops is someone opens a UBI device sysfs file,
then the device is removed, and he reads the file. With this fix, he
will get -ENODEV instead of an oops.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/build.c

@@ -263,8 +263,12 @@ static ssize_t dev_attribute_show(struct device *dev,
 	return ret;
 }
 
-/* Fake "release" method for UBI devices */
-static void dev_release(struct device *dev) { }
+static void dev_release(struct device *dev)
+{
+	struct ubi_device *ubi = container_of(dev, struct ubi_device, dev);
+
+	kfree(ubi);
+}
 
 /**
  * ubi_sysfs_init - initialize sysfs for an UBI device.
@@ -944,6 +948,12 @@ int ubi_detach_mtd_dev(int ubi_num, int anyway)
 	if (ubi->bgt_thread)
 		kthread_stop(ubi->bgt_thread);
 
+	/*
+	 * Get a reference to the device in order to prevent 'dev_release()'
+	 * from freeing @ubi object.
+	 */
+	get_device(&ubi->dev);
+
 	uif_close(ubi);
 	ubi_wl_close(ubi);
 	free_internal_volumes(ubi);
@@ -955,7 +965,7 @@ int ubi_detach_mtd_dev(int ubi_num, int anyway)
 	vfree(ubi->dbg_peb_buf);
 #endif
 	ubi_msg("mtd%d is detached from ubi%d", ubi->mtd->index, ubi->ubi_num);
-	kfree(ubi);
+	put_device(&ubi->dev);
 	return 0;
 }