m68k: fix __page_to_pfn for a const struct page argument

Fixes fallout due to the removal of the cast in commit aa462abe8aaf
("mm: fix __page_to_pfn for a const struct page argument")

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/m68k/include/asm/page_mm.h

@@ -162,7 +162,7 @@ static inline __attribute_const__ int __virt_to_node_shift(void)
 	pgdat->node_mem_map + (__pfn - pgdat->node_start_pfn);		\
 })
 #define page_to_pfn(_page) ({						\
-	struct page *__p = (_page);					\
+	const struct page *__p = (_page);				\
 	struct pglist_data *pgdat;					\
 	pgdat = &pg_data_map[page_to_nid(__p)];				\
 	((__p) - pgdat->node_mem_map) + pgdat->node_start_pfn;		\