CVE-2018-3639 (powerpc)

We might have migrated to a machine that uses a different flush type,
or doesn't need flushing at all.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from 921bc6cf)
[mauricio: backport: pseries.h: update context lines]
Signed-off-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>