mkfs.f2fs: fix to calculate left space of checkpoint page correctly
mkfs.f2fs: fix to calculate left space of checkpoint page correctly

We reserved one segment for NAT region at least, so when formatting
fs, calculated maximum size of left space in CP page should be:
CHECKSUM_OFFSET - sizeof(struct f2fs_checkpoint) + 1 - 64 (size of
reserved NAT bitmap).

Fix the incorrect calculated size to avoid potential overflow bug here.

Signed-off-by: Chao Yu <0cc5db7488e13b393f245aa6263acec5e92fdcc5@samsung.com>
Signed-off-by: Jaegeuk Kim <71255387a32c360dfbe6dfb7aaeb123bf1dc8cb6@kernel.org>

TurboROM/external_f2fs-tools,TurboROM/external_f2fs-tools