split: allow writing to symlink to nonseekable

[oech3]

Fixes #13409

[codspeed-hq]

Merging this PR will not alter performance

✅ 339 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing oech3:split-stream (6851841) with main (686ce09)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/pid-pipe (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/sparse-to-pipe is no longer failing!
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!