UBIFS: Fix potential integer overflow in allocation

There is potential for the size and len fields in ubifs_data_node to be
too large causing either a negative value for the length fields or an
integer overflow leading to an incorrect memory allocation. Likewise,
when the len field is small, an integer underflow may occur.

Signed-off-by: Silvio Cesare <silvio.cesare@gmail.com>
Fixes: 1e51764a ("UBIFS: add new flash file system")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>