UBI: Validate data_size

[ Upstream commit 281fda27673f833a01d516658a64d22a32c8e072 ]

Make sure that data_size is less than LEB size.
Otherwise a handcrafted UBI image is able to trigger
an out of bounds memory access in ubi_compare_lebs().

Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: David Gstir <david@sigma-star.at>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>