pstore: Fix flags to enable dumps on powerpc

pstore: Fix flags to enable dumps on powerpc

BugLink: http://bugs.launchpad.net/bugs/1691045
After commit c950fd6f201a kernel registers pstore write based on flag set.
Pstore write for powerpc is broken as flags(PSTORE_FLAGS_DMESG) is not set for
powerpc architecture. On panic, kernel doesn't write message to
/fs/pstore/dmesg*(Entry doesn't gets created at all).

This patch enables pstore write for powerpc architecture by setting
PSTORE_FLAGS_DMESG flag.

Fixes: c950fd6f201a ("pstore: Split pstore fragile flags")
Cc: stable@vger.kernel.org # v4.9+
Signed-off-by: Ankit Kumar <ankit@linux.vnet.ibm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
(cherry picked from commit 041939c1ec54208b42f5cd819209173d52a29d34)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Acked-by: Colin King <colin.king@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>