mm/sl{a,u}b: fix wrong usages of folio_page() for getting head pages

mm/sl{a,u}b: fix wrong usages of folio_page() for getting head pages

The standard idiom for getting head page of a given folio is
'&folio->page', but some are wrongly using 'folio_page(folio, 0)' for
the purpose.  Fix those to use the idiom.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>