powerpc: hard disable irqs in smp_send_stop loop

powerpc: hard disable irqs in smp_send_stop loop

BugLink: http://bugs.launchpad.net/bugs/1790636
The hard lockup watchdog can fire under local_irq_disable
on platforms with irq soft masking.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 855bfe0de1a05a01f89975ea8ba9f5521fb0f567)
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>