mm: make HASHED_PAGE_VIRTUAL page_address' struct page argument const.

Followup to 33dd4e0ec911 "mm: make some struct page's const" which missed the
HASHED_PAGE_VIRTUAL case.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/hash.h b/include/linux/hash.h
index 06d25c1..b80506b 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -63,7 +63,7 @@
 	return hash >> (32 - bits);
 }
 
-static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
+static inline unsigned long hash_ptr(const void *ptr, unsigned int bits)
 {
 	return hash_long((unsigned long)ptr, bits);
 }