commit 7e467245 upstream.

We would get wrong results in compiler recomputed old_pmd. Avoid
that by using ACCESS_ONCE
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>