fs: fix flush regression from fast path

[ChALkeR]

Fixes: #60552

Regression from: #49884

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.55%. Comparing base (dbe45b7) to head (5142f4e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60553      +/-   ##
==========================================
+ Coverage   88.54%   88.55%   +0.01%     
==========================================
  Files         704      704              
  Lines      207843   207853      +10     
  Branches    40044    40056      +12     
==========================================
+ Hits       184028   184069      +41     
+ Misses      15858    15802      -56     
- Partials     7957     7982      +25     

Files with missing lines	Coverage Δ
lib/fs.js	98.19% <100.00%> (+<0.01%)	⬆️

... and 37 files with indirect coverage changes

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