overlay: correct fsck.overlay exit code

overlay: correct fsck.overlay exit code

fsck.overlay should return correct exit code to show the file system
status after fsck, instead of return 0 means consistency and !0 means
inconsistency or something bad happened.

Fix the following three exit code after running fsck.overlay:

- Return FSCK_OK if the input file system is consistent,
- Return FSCK_NONDESTRUCT if the file system inconsistent errors
  corrected,
- Return FSCK_UNCORRECTED if the file system still have inconsistent
  errors.

This patch also add a helper function to run fsck.overlay and check
the return value is expected or not.

[amir] rename helper to _overlay_fsck_expect, split define of FSCK_*
to a seprate path.

Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>