Now when madvise(MADV_REMOVE) notifies uffd reader, we should verify
that appliciation actually sees zeros at the removed range.

Link: http://lkml.kernel.org/r/1484814154-1557-4-git-send-email-rppt@linux.vnet.ibm.comSigned-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>