SSTFileWriter don't report file size when enabled zstd dictionary training

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I was trying to limit the file size when bulk loading with SSTFileWriter, and I find the `FileSize()` always return `0` when zstd dictionary training is enabled.

### Expected behavior

`sstFileWriter.FileSize()` should report current progress.

### Actual behavior

`sstFileWriter.FileSize()` always return `0` when zstd dictionary training is enabled.

### Steps to reproduce the behavior