mm: Add lru_to_folio()

Since page->lru occupies the same bytes as compound_head, any page
on the LRU list must be a folio.

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

include/linux/mm.h

@@ -225,6 +225,10 @@ int overcommit_policy_handler(struct ctl_table *, int, void *, size_t *,
 #define PAGE_ALIGNED(addr)	IS_ALIGNED((unsigned long)(addr), PAGE_SIZE)
 
 #define lru_to_page(head) (list_entry((head)->prev, struct page, lru))
+static inline struct folio *lru_to_folio(struct list_head *head)
+{
+	return list_entry((head)->prev, struct folio, lru);
+}
 
 void setup_initial_init_mm(void *start_code, void *end_code,
 			   void *end_data, void *brk);