mm: Reduce the number of slab->folio casts

Mark a few more folio functions as taking a const folio pointer, which
allows us to remove a few places in slab which cast away the const.

Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Signed-off-by: Vlastimil Babka <[e-mail]>

include/linux/mm.h

···
  *
  * Return: The order of the folio.
  */
-static inline unsigned int folio_order(struct folio *folio)
+static inline unsigned int folio_order(const struct folio *folio)
 {
 	if (!folio_test_large(folio))
 		return 0;
···
  * it from being split.  It is not necessary for the folio to be locked.
  * Return: The base-2 logarithm of the size of this folio.
  */
-static inline unsigned int folio_shift(struct folio *folio)
+static inline unsigned int folio_shift(const struct folio *folio)
 {
 	return PAGE_SHIFT + folio_order(folio);
 }
···
  * it from being split.  It is not necessary for the folio to be locked.
  * Return: The number of bytes in this folio.
  */
-static inline size_t folio_size(struct folio *folio)
+static inline size_t folio_size(const struct folio *folio)
 {
 	return PAGE_SIZE << folio_order(folio);
 }

mm/slab.h

···
  */
 static inline bool slab_test_pfmemalloc(const struct slab *slab)
 {
-	return folio_test_active((struct folio *)slab_folio(slab));
+	return folio_test_active(slab_folio(slab));
 }
 
 static inline void slab_set_pfmemalloc(struct slab *slab)
···
 
 static inline int slab_order(const struct slab *slab)
 {
-	return folio_order((struct folio *)slab_folio(slab));
+	return folio_order(slab_folio(slab));
 }
 
 static inline size_t slab_size(const struct slab *slab)

mm/slub.c

···
 
 static void print_slab_info(const struct slab *slab)
 {
-	struct folio *folio = (struct folio *)slab_folio(slab);
-
 	pr_err("Slab 0x%p objects=%u used=%u fp=0x%p flags=%pGp\n",
 	       slab, slab->objects, slab->inuse, slab->freelist,
-	       folio_flags(folio, 0));
+	       &slab->__page_flags);
 }
 
 /*
···
  */
 static inline bool slab_test_node_partial(const struct slab *slab)
 {
-	return folio_test_workingset((struct folio *)slab_folio(slab));
+	return folio_test_workingset(slab_folio(slab));
 }
 
 static inline void slab_set_node_partial(struct slab *slab)