UBI: Validate data_size

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().

CRs-Fixed: 975289
Change-Id: I1edf4c18028668e744059d42da8b7c298f5f640e
Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: David Gstir <david@sigma-star.at>
Git-commit: 281fda27673f833a01d516658a64d22a32c8e072
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>