f2fs: rebuild nat_bits during umount

If all free_nat_bitmap are available, we can rebuild nat_bits from
free_nat_bitmap entirely during umount, let's make another chance
to reenable nat_bits for image.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>