slab: Remove folio references from kfree()

This should generate identical code to the previous version, but
without any dependency on how folios work.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20251113000932.1589073-9-willy@infradead.org
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: Matthew Wilcox (Oracle)

mm/slub.c

@@ -6817,7 +6817,7 @@ void kvfree_rcu_cb(struct rcu_head *head)
  */
 void kfree(const void *object)
 {
-	struct folio *folio;
+	struct page *page;
 	struct slab *slab;
 	struct kmem_cache *s;
 	void *x = (void *)object;
@@ -6827,13 +6827,13 @@ void kfree(const void *object)
 	if (unlikely(ZERO_OR_NULL_PTR(object)))
 		return;
 
-	folio = virt_to_folio(object);
-	if (unlikely(!folio_test_slab(folio))) {
-		free_large_kmalloc(&folio->page, (void *)object);
+	page = virt_to_page(object);
+	slab = page_slab(page);
+	if (!slab) {
+		free_large_kmalloc(page, (void *)object);
 		return;
 	}
 
-	slab = folio_slab(folio);
 	s = slab->slab_cache;
 	slab_free(s, slab, x, _RET_IP_);
 }