MIPS: smp: Make stop_this_cpu() actually stop the CPU

Since cpu_wait() enables interrupts upon return, CPUs which have
entered stop_this_cpu() may still end up handling interrupts.
This can lead to the softlockup detector firing on a panic or
restart/poweroff/halt.  Just disable interrupts and spin to ensure
nothing else runs on the CPU once it has entered stop_this_cpu().

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9601/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/smp.c

@@ -176,10 +176,8 @@ static void stop_this_cpu(void *dummy)
 	 * Remove this CPU:
 	 */
 	set_cpu_online(smp_processor_id(), false);
-	for (;;) {
-		if (cpu_wait)
-			(*cpu_wait)();		/* Wait if available. */
-	}
+	local_irq_disable();
+	while (1);
 }
 
 void smp_send_stop(void)