fs: expose frsize field in statfs

[verycosy]

Expose f_frsize from libuv's uv_statfs_t as statfs.frsize.

libuv 1.52.0 added f_frsize to uv_statfs_t, and this dependency update has already landed in Node.js (5bebd7e),
so users can now compute accurate disk usage on every filesystem type.

Per POSIX, f_blocks, f_bfree, and f_bavail are in units of f_frsize, not f_bsize. On most filesystems the two values happen to be equal, so blocks * bsize gives a correct result. However, some filesystem drivers report a larger bsize as the I/O transfer size while frsize remains at the actual block size, producing a significant overestimation.

I hit this on a Docker container using virtiofs with a bind-mounted host directory:

$ node -e "console.log(fs.statfsSync('/data'))"
StatFs {
  bsize: 1048576,   // 1 MiB — I/O transfer size
  frsize: 4096,     // 4 KiB — actual block size
  blocks: 120699413,
  ...
}

Calculation	Result
blocks × frsize	~460 GB	correct
blocks × bsize	~117,870 GB	256× overestimation

Without frsize exposed, there is no way to work around this using the Node.js API alone.

See also libuv/libuv#4983 where the same problem was discussed.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71807/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.67%. Comparing base (4579957) to head (c848da3).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #62277   +/-   ##
=======================================
  Coverage   89.67%   89.67%           
=======================================
  Files         676      676           
  Lines      206555   206557    +2     
  Branches    39554    39546    -8     
=======================================
+ Hits       185230   185236    +6     
+ Misses      13461    13444   -17     
- Partials     7864     7877   +13     

Files with missing lines	Coverage Δ
lib/internal/fs/utils.js	99.68% <100.00%> (+<0.01%)	⬆️
src/node_file-inl.h	87.71% <100.00%> (+0.05%)	⬆️
src/node_file.h	78.43% <ø> (ø)

... and 31 files with indirect coverage changes

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