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: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>