Fix VLA warning in io_threads

[yang-z-o]

#3324 introduced BATCH_SIZE as a const int local variable and used it as an array bound. Clang 17 rejects this with:

io_threads.c:305:22: error: variable length array folded to constant array as an extension [-Werror,-Wgnu-folding-constant]
  305 |     void *batch_jobs[BATCH_SIZE];
      |                      ^~~~~~~~~~
1 error generated.
make[1]: *** [io_threads.o] Error 1
make: *** [all] Error 2

Old Clang versions do not emit this warning, maybe that is why the CI passed. Fix by promoting BATCH_SIZE to a file-scope #define.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.62%. Comparing base (087280e) to head (113aefd).
⚠️ Report is 9 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #3518      +/-   ##
============================================
+ Coverage     76.47%   76.62%   +0.15%     
============================================
  Files           159      159              
  Lines         79840    79839       -1     
============================================
+ Hits          61060    61180     +120     
+ Misses        18780    18659     -121     

Files with missing lines	Coverage Δ
src/io_threads.c	70.52% <ø> (-0.29%)	⬇️

... and 21 files with indirect coverage changes

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[yang-z-o]

Found a similar fix #3439

[roshkhatri]

LGTM