zsmalloc: use page->private instead of page->first_page

 * struct page(s) to form a zspage.

 *

 * Usage of struct page fields:

 *	page->first_page: points to the first component (0-order) page

 *	page->private: points to the first component (0-order) page

 *	page->index (union with page->freelist): offset of the first object

 *		starting in this page. For the first page, this is

 *		always 0, so we use this field (aka freelist) to point

 *

 *	For _first_ page only:

 *

 *	page->private (union with page->first_page): refers to the

 *		component page after the first page

 *	page->private: refers to the component page after the first page

 *		If the page is first_page for huge object, it stores handle.

 *		Look at size_class->huge.

 *	page->freelist: points to the first free object in zspage.

	if (is_first_page(page))

		return page;

	else

		return page->first_page;

		return (struct page *)page_private(page);

}

static struct page *get_next_page(struct page *page)

	 * Allocate individual pages and link them together as:

	 * 1. first page->private = first sub-page

	 * 2. all sub-pages are linked together using page->lru

	 * 3. each sub-page is linked to the first page using page->first_page

	 * 3. each sub-page is linked to the first page using page->private

	 *

	 * For each size class, First/Head pages are linked together using

	 * page->lru. Also, we set PG_private to identify the first page

		if (i == 1)

			set_page_private(first_page, (unsigned long)page);

		if (i >= 1)

			page->first_page = first_page;

			set_page_private(page, (unsigned long)first_page);

		if (i >= 2)

			list_add(&page->lru, &prev_page->lru);

		if (i == class->pages_per_zspage - 1)	/* last page */