target: use vfs_iter_read/write in fd_do_rw

	struct fd_dev *dev = FD_DEV(se_dev);

	struct file *fd = dev->fd_file;

	struct scatterlist *sg;

	struct iovec *iov;

	mm_segment_t old_fs;

	struct iov_iter iter;

	struct bio_vec *bvec;

	ssize_t len = 0;

	loff_t pos = (cmd->t_task_lba * se_dev->dev_attrib.block_size);

	int ret = 0, i;

	iov = kzalloc(sizeof(struct iovec) * sgl_nents, GFP_KERNEL);

	if (!iov) {

	bvec = kcalloc(sgl_nents, sizeof(struct bio_vec), GFP_KERNEL);

	if (!bvec) {

		pr_err("Unable to allocate fd_do_readv iov[]\n");

		return -ENOMEM;

	}

	for_each_sg(sgl, sg, sgl_nents, i) {

		iov[i].iov_len = sg->length;

		iov[i].iov_base = kmap(sg_page(sg)) + sg->offset;

	}

		bvec[i].bv_page = sg_page(sg);

		bvec[i].bv_len = sg->length;

		bvec[i].bv_offset = sg->offset;

	old_fs = get_fs();

	set_fs(get_ds());

		len += sg->length;

	}

	iov_iter_bvec(&iter, ITER_BVEC, bvec, sgl_nents, len);

	if (is_write)

		ret = vfs_writev(fd, &iov[0], sgl_nents, &pos);

		ret = vfs_iter_write(fd, &iter, &pos);

	else

		ret = vfs_readv(fd, &iov[0], sgl_nents, &pos);

	set_fs(old_fs);

	for_each_sg(sgl, sg, sgl_nents, i)

		kunmap(sg_page(sg));

		ret = vfs_iter_read(fd, &iter, &pos);

	kfree(iov);

	kfree(bvec);

	if (is_write) {

		if (ret < 0 || ret != cmd->data_length) {