fs: use ftruncate on OpenBSD in AllocateFileRange #32643

1. 
   fanquake commented at 9:17 am on May 30, 2025: member

   OpenBSD doesn’t have fallocate or posix_fallocate, which means it’s hitting our fallback. We should instead have it use ftruncate. See related discussion in #32460.

   cc @theStack.

2. fanquake added the label Utils/log/libs on May 30, 2025

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