Casting operands to a wider type to prevent integer overflow

Casting operands to a wider type to prevent integer overflow

Previously the calculation for total_size was done in 32-bits, as the
operands were all 32-bit signed or unsigned integers. This led to
integer overflow when the extent being allocated is too large.
total_size is finally cast to size_t, as the kernel UAPI for ION uses
size_t.

Change-Id: I7a76b2c18be25fda0bf6ef70cd8a6fe717c2903c
Signed-off-by: Fufu Fang <fufu.fang@arm.com>