mm: make some struct page's const

These uses are read-only and in a subsequent patch I have a const struct
page in my hand...

[[e-mail]: fix warnings in lowmem_page_address()]
Signed-off-by: Ian Campbell <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Michel Lespinasse <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/mm.h

···
 #define SECTIONS_MASK		((1UL << SECTIONS_WIDTH) - 1)
 #define ZONEID_MASK		((1UL << ZONEID_SHIFT) - 1)
 
-static inline enum zone_type page_zonenum(struct page *page)
+static inline enum zone_type page_zonenum(const struct page *page)
 {
 	return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK;
 }
···
 }
 
 #ifdef NODE_NOT_IN_PAGE_FLAGS
-extern int page_to_nid(struct page *page);
+extern int page_to_nid(const struct page *page);
 #else
-static inline int page_to_nid(struct page *page)
+static inline int page_to_nid(const struct page *page)
 {
 	return (page->flags >> NODES_PGSHIFT) & NODES_MASK;
 }
 #endif
 
-static inline struct zone *page_zone(struct page *page)
+static inline struct zone *page_zone(const struct page *page)
 {
 	return &NODE_DATA(page_to_nid(page))->node_zones[page_zonenum(page)];
 }
···
  */
 #include <linux/vmstat.h>
 
-static __always_inline void *lowmem_page_address(struct page *page)
+static __always_inline void *lowmem_page_address(const struct page *page)
 {
-	return __va(PFN_PHYS(page_to_pfn(page)));
+	return __va(PFN_PHYS(page_to_pfn((struct page *)page)));
 }
 
 #if defined(CONFIG_HIGHMEM) && !defined(WANT_PAGE_VIRTUAL)

mm/sparse.c

···
 static u16 section_to_node_table[NR_MEM_SECTIONS] __cacheline_aligned;
 #endif
 
-int page_to_nid(struct page *page)
+int page_to_nid(const struct page *page)
 {
 	return section_to_node_table[page_to_section(page)];
 }