ubiformat: fix failure on big partitions (>4Gio)

ubiformat: fix failure on big partitions (>4Gio)

The offset (which is 64bits when mtd-utils are not compile with
WITHOUT_LARGEFILE) is calculated like that:
offset = nb * size;
But nb and size are int, so on 32bits platforms, there's a possible
overflow.

So, it should be replace with:
offset = (off_t)nb * size;
If WITHOUT_LARGEFILE is defined, there still be an overflow, but it's
what we want, right ?

Cheney Chen tested an ubiformat on a NAND (5.9 GiB mtd part).

Reported-by: Cheney Chen <cheneychencl2012@gmail.com>
Tested-by: Cheney Chen <cheneychencl2012@gmail.com>
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>