hugetlb: fix clear_user_highpage arguments

The virtual address space argument of clear_user_highpage is supposed to be
the virtual address where the page being cleared will eventually be mapped.
 This allows architectures with virtually indexed caches a few clever
tricks.  That sort of trick falls over in painful ways if the virtual
address argument is wrong.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>