slab: Remove references to folios from virt_to_slab()

Use page_slab() instead of virt_to_folio() which will work
perfectly when struct slab is separated from struct folio.

This was the last user of folio_slab(), so delete it.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20251113000932.1589073-17-willy@infradead.org
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: Matthew Wilcox (Oracle)

mm/slab.h

@@ -117,19 +117,6 @@ static_assert(sizeof(struct slab) <= sizeof(struct page));
 static_assert(IS_ALIGNED(offsetof(struct slab, freelist), sizeof(freelist_aba_t)));
 #endif
 
-/**
- * folio_slab - Converts from folio to slab.
- * @folio: The folio.
- *
- * Currently struct slab is a different representation of a folio where
- * folio_test_slab() is true.
- *
- * Return: The slab which contains this folio.
- */
-#define folio_slab(folio)	(_Generic((folio),			\
-	const struct folio *:	(const struct slab *)(folio),		\
-	struct folio *:		(struct slab *)(folio)))
-
 /**
  * slab_folio - The folio allocated for a slab
  * @s: The slab.
@@ -192,12 +179,7 @@ static inline pg_data_t *slab_pgdat(const struct slab *slab)
 
 static inline struct slab *virt_to_slab(const void *addr)
 {
-	struct folio *folio = virt_to_folio(addr);
-
-	if (!folio_test_slab(folio))
-		return NULL;
-
-	return folio_slab(folio);
+	return page_slab(virt_to_page(addr));
 }
 
 static inline int slab_order(const struct slab *slab)