powerpc/64: hard disable irqs in panic_smp_self_stop

Similarly to commit 855bfe0de1 ("powerpc: hard disable irqs in
smp_send_stop loop"), irqs should be hard disabled by
panic_smp_self_stop.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/setup_64.c

@@ -387,6 +387,14 @@ void early_setup_secondary(void)
 
 #endif /* CONFIG_SMP */
 
+void panic_smp_self_stop(void)
+{
+	hard_irq_disable();
+	spin_begin();
+	while (1)
+		spin_cpu_relax();
+}
+
 #if defined(CONFIG_SMP) || defined(CONFIG_KEXEC_CORE)
 static bool use_spinloop(void)
 {