Method fsync

fs/xfile.cpp:67–72

            return pio("pwrite", _and_pwrite(), (void*)buf, count, offset);
        }
        virtual int fsync() override
        {
            for (auto x: m_files)
                x->fsync();
            return 0;
        }
        virtual int fdatasync() override
        {
            for (auto x: m_files)