Fix possible data race when committing block files

It was recently pointed out to me that calling fsync() or fdatasync() on a new
file is not sufficient to ensure it's persisted to disk, a the existence of the
file itself is stored in the directory inode. This means that ensuring that a
new file is actually committed also requires an fsync() on the parent directory.

This change ensures that we call fsync() on the blocks directory after
committing new block files.

Author: eklitzke

src/flatfile.cpp

@@ -92,6 +92,7 @@ bool FlatFileSeq::Flush(const FlatFilePos& pos, bool finalize)
         fclose(file);
         return error("%s: failed to commit file %d", __func__, pos.nFile);
     }
+    DirectoryCommit(m_dir);
 
     fclose(file);
     return true;