mm: constify page_is_pfmemalloc() argument

The function only tests for page->index, so its argument should be
const.
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/mm.h

@@ -1584,7 +1584,7 @@ struct address_space *page_mapping_file(struct page *page);
  * ALLOC_NO_WATERMARKS and the low watermark was not
  * met implying that the system is under some pressure.
  */
-static inline bool page_is_pfmemalloc(struct page *page)
+static inline bool page_is_pfmemalloc(const struct page *page)
 {
 	/*
 	 * Page index cannot be this large so this must be