f2fs: avoid value overflow in showing current status

This patch fixes overflow when do cat /sys/kernel/debug/f2fs/status.
If a section is relatively large, dist value can be overflowed.

Reported-by: Yossi Goldfill <ygoldfill@radianmemory.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>