fuse: verify zero padding in fuse_backing_map

To allow us extending the interface in the future.

Fixes: 4435025 ("fuse: implement ioctls to manage backing files")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/passthrough.c

@@ -225,7 +225,7 @@ int fuse_backing_open(struct fuse_conn *fc, struct fuse_backing_map *map)
 		goto out;
 
 	res = -EINVAL;
-	if (map->flags)
+	if (map->flags || map->padding)
 		goto out;
 
 	file = fget(map->fd);