Revert "UBIFS: Fix potential integer overflow in allocation"

This reverts commit 353748a3.
It bypassed the linux-mtd review process and fixes the issue not as it
should.

Cc: Kees Cook <keescook@chromium.org>
Cc: Silvio Cesare <silvio.cesare@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>