/* this function will write some specified length data to file system. */
ssize_t dfs_file_write(struct dfs_file *fd, const void *buf, size_t len)
/* this function will write some specified length data to file system. */
ssize_t dfs_file_write(struct dfs_file *fd, const void *buf, size_t len)
{
    if (fd == NULL)
    {
        return -EINVAL;
    }
    if (fd->vnode->fops->write == NULL)
    {
        return -ENOSYS;
    }
    return fd->vnode->fops->write(fd, buf, len);
}