UPSTREAM: mm/page_poison: expose page_poisoning_enabled to kernel modules

In some usages, e.g. virtio-balloon, a kernel module needs to know if
page poisoning is in use. This patch exposes the page_poisoning_enabled
function to kernel modules.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Change-Id: Icab2f64ef17850ac8b636bd302653bb5f1960e8c
(cherry picked from d95f58f4a6ca002126c36c530fb096519c94baef)
Signed-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Todd Kjos <tkjos@google.com>