ubifs: Reject unsupported ioctl flags explicitly

Reject unsupported ioctl flags explicitly, so the following command
on a regular ubifs file will fail:
	chattr +d ubifs_file

And xfstests generic/424 will pass.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>