fs: use correct position when retrying writev

[christianaurichzm]

WriteStream retries a partial writev() at the wrong position when start is set.

writevAll() tracks the next position in pos, but passes this.pos to writev(). _writev() advances this.pos by the full batch size before the callback runs, so a short write can retry from the end of the batch instead of after the bytes that were written.

For a 12-byte batch starting at 0, with the first call writing 3 bytes:

writev at 0
writev at 12

The second call should be at 3. The current result is:

"AAA\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000ABBBBCCCC"

EAGAIN has the same problem: the retry uses the advanced this.pos even though no bytes were written.

Pass pos instead, matching writeAll().

The regression test covers partial writes at start: 0 and start: 5, EAGAIN, and the unpositioned case.

For start: 5:

partial write: [5, 17] -> [5, 8]
EAGAIN:        [5, 17] -> [5, 5]

make lint passes. parallel and sequential were also run on Linux x64. Four parallel failures related to --use-openssl-ca also reproduce on the base commit.

Refs: #49211

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (36dd044) to head (f6535a4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #66203   +/-   ##
=======================================
  Coverage   90.29%   90.29%           
=======================================
  Files         790      790           
  Lines      272529   272529           
  Branches    52031    52033    +2     
=======================================
+ Hits       246083   246089    +6     
+ Misses      16909    16897   -12     
- Partials     9537     9543    +6     

Files with missing lines	Coverage Δ
lib/internal/fs/streams.js	94.45% <100.00%> (+1.34%)	⬆️

... and 26 files with indirect coverage changes

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