fs: return sorted entries for ReaddirPlus when sort-dir is true

[jiefenghuang]

ref: #6442

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.16%. Comparing base (2d372c7) to head (e9ccf91).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/fs/fs.go	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6468      +/-   ##
==========================================
+ Coverage   48.11%   51.16%   +3.05%     
==========================================
  Files         133      167      +34     
  Lines       42009    51077    +9068     
==========================================
+ Hits        20213    26136    +5923     
- Misses      19445    22011    +2566     
- Partials     2351     2930     +579     

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