[PATCH,HURD] implement syncfs

[Christoph Hellwig]

> +/* Make all changes done to all files on the file system associated
> +   with FD actually appear on disk.  */
> +int
> +syncfs (int fd)
> +{
> +  /* This is not actually synchronous; we don't wait.  */
> +  error_t err = HURD_DPORT_USE (fd, __file_syncfs (port, 0, 0));

Note that syncfs in Linux actually does wait for completion of the writeback,
that's the whole point of the syscall.