kobject: Restore old behaviour of kobject_del(NULL)

The commit 079ad2fb ("kobject: Avoid premature parent object freeing in
kobject_cleanup()") inadvertently dropped a possibility to call kobject_del()
with NULL pointer. Restore the old behaviour.

Fixes: 079ad2fb ("kobject: Avoid premature parent object freeing in kobject_cleanup()")
Cc: stable <stable@vger.kernel.org>
Reported-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Link: https://lore.kernel.org/r/20200803082706.65347-1-andriy.shevchenko@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/kobject.c

@@ -637,8 +637,12 @@ static void __kobject_del(struct kobject *kobj)
  */
 void kobject_del(struct kobject *kobj)
 {
-	struct kobject *parent = kobj->parent;
+	struct kobject *parent;
+
+	if (!kobj)
+		return;
 
+	parent = kobj->parent;
 	__kobject_del(kobj);
 	kobject_put(parent);
 }