fsck.f2fs: write checkpoint with OPU mode

fsck.f2fs: write checkpoint with OPU mode

This original patch was from Weichao Guo.

We may encounter both checkpoints invalid in such a case:
1. kernel writes CP A;
2. power-cut when kernel writes CP B, then CP B is corrupted;
3. fsck: load CP A, fix meta/data;
4. power-cut when fsck writes CP A in-place, then CP A is corrupted too;

To avoid both checkpoints being invalid, this patch changes to duplicate
valid checkpoint to mirror position first, and then, write fixed checkpoint
to CP #0 position.

This can make sure that, while fsck repairing, even there is sudden
power-cut, last valid checkpoint can be kept in CP #1 position.

Signed-off-by: Weichao Guo <guoweichao@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>