f2fs: fix to check available space of CP area correctly in update_ckpt_flags()

Otherwise, nat_bit area may be persisted across boundary of CP area during
nat_bit rebuilding.

Fixes: 94c821fb ("f2fs: rebuild nat_bits during umount")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>