fuse_ioctl_copy_user(): don't open-code copy_page_{to,from}_iter()

	while (iov_iter_count(&ii)) {

		struct page *page = pages[page_idx++];

		size_t todo = min_t(size_t, PAGE_SIZE, iov_iter_count(&ii));

		void *kaddr;

		kaddr = kmap(page);

		while (todo) {

			char __user *uaddr = ii.iov->iov_base + ii.iov_offset;

			size_t iov_len = ii.iov->iov_len - ii.iov_offset;

			size_t copy = min(todo, iov_len);

			size_t left;

		size_t copied;

		if (!to_user)

				left = copy_from_user(kaddr, uaddr, copy);

			copied = copy_page_from_iter(page, 0, PAGE_SIZE, &ii);

		else

				left = copy_to_user(uaddr, kaddr, copy);

			copied = copy_page_to_iter(page, 0, PAGE_SIZE, &ii);

			if (unlikely(left))

		if (unlikely(copied != PAGE_SIZE && iov_iter_count(&ii)))

			return -EFAULT;

			iov_iter_advance(&ii, copy);

			todo -= copy;

			kaddr += copy;

		}

		kunmap(page);

	}

	return 0;