mm: fixup compilation error due to an asm write through a const pointer

This regression has been introduced in

commit f56f821f
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Mar 25 19:47:41 2012 +0200

    mm: extend prefault helpers to fault in more than PAGE_SIZE

I have failed to notice this because x86 asm seems to happily compile
things as-is.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NDave Airlie <airlied@redhat.com>

include/linux/pagemap.h

@@ -461,7 +461,7 @@ static inline int fault_in_pages_readable(const char __user *uaddr, int size)
 static inline int fault_in_multipages_writeable(char __user *uaddr, int size)
 {
 	int ret;
-	const char __user *end = uaddr + size - 1;
+	char __user *end = uaddr + size - 1;
 
 	if (unlikely(size == 0))
 		return 0;