hugetlb: call mmu notifiers on hugepage cow

When a copy-on-write occurs, we take one of two paths in handle_mm_fault:
through handle_pte_fault for normal pages, or through hugetlb_fault for
huge pages.

In the normal page case, we eventually get to do_wp_page and call mmu
notifiers via ptep_clear_flush_notify.  There is no callout to the mmmu
notifiers in the huge page case.  This patch fixes that.

Signed-off-by: Doug Doan <dougd@cray.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>