p_fallocate: compatibility fixes for macOS

On macOS, fcntl(..., F_PREALLOCATE, ...) will only succeed when followed
by an ftruncate(), even when it reports success.

However, that syscall will fail when the file already exists.  Thus, we
must ignore the error code and simply let ftruncate extend the size of
the file itself (albeit slowly).

By calling ftruncate, we also need to prevent against file shrinkage,
for compatibility with posix_ftruncate, which will only extend files,
never shrink them.

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