zsmalloc: Stop using slab fields in struct page

zsmalloc: Stop using slab fields in struct page

The ->freelist and ->units members of struct page are for the use of slab
only.  I'm not particularly familiar with zsmalloc, so generate the same
code by using page->index to store 'page' (page->index and page->freelist
are at the same offset in struct page).  This should be cleaned up
properly at some point by somebody who is familiar with zsmalloc.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>