f2fs: avoid an overflow during utilization calculation

The current f2fs uses all the block counts with 32 bit numbers, which is able to
cover about 15TB volume.

But in calculation of utilization, f2fs multiplies the count by 100 which can
induce overflow.
This patch fixes this.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>