[WIP]nydusd: support cas gc in umount

[Desiki-high]

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Details

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Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.10%. Comparing base (375f55f) to head (11e9d75).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1639      +/-   ##
==========================================
- Coverage   60.10%   60.10%   -0.01%     
==========================================
  Files         147      147              
  Lines       49513    49513              
  Branches    46994    46994              
==========================================
- Hits        29762    29760       -2     
- Misses      17970    17971       +1     
- Partials     1781     1782       +1     

see 2 files with indirect coverage changes