vsprintf:fix: Change type returned by ustrtoul

The ustrtoul shall convert string defined size (e.g. 1GiB) to unsigned
long type (as its name implies).

Up till now it had returned int, which might cause problems with large
numbers (GiB range), when interpreted as U2 signed numbers.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>