mm/hmm: constify hmm_devmem_page_get_drvdata() parameter

Constify pointer parameter to avoid issue when use from code that only
has const struct page pointer to use in the first place.

Link: http://lkml.kernel.org/r/1506972774-10191-1-git-send-email-jglisse@redhat.com
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/hmm.h

@@ -471,9 +471,9 @@ static inline void hmm_devmem_page_set_drvdata(struct page *page,
  * @page: pointer to struct page
  * Return: driver data value
  */
-static inline unsigned long hmm_devmem_page_get_drvdata(struct page *page)
+static inline unsigned long hmm_devmem_page_get_drvdata(const struct page *page)
 {
-	unsigned long *drvdata = (unsigned long *)&page->pgmap;
+	const unsigned long *drvdata = (const unsigned long *)&page->pgmap;
 
 	return drvdata[1];
 }