powerpc/pmac: Fix SMP kernels on pre-core99 UP machines

commit 78c5c68a upstream.

The code for "powersurge" SMP would kick in and cause a crash
at boot due to the lack of a NULL test.

Adam Conrad reports that the 3.2 kernel, with CONFIG_SMP=y, will not
boot on an OldWorld G3; we're unconditionally writing to psurge_start,
but this is only set on powersurge machines.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Reported-by: Adam Conrad <adconrad@ubuntu.com>
Tested-by: Adam Conrad <adconrad@ubuntu.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>

arch/powerpc/platforms/powermac/smp.c

@@ -402,7 +402,7 @@ static struct irqaction psurge_irqaction = {
 
 static void __init smp_psurge_setup_cpu(int cpu_nr)
 {
-	if (cpu_nr != 0)
+	if (cpu_nr != 0 || !psurge_start)
 		return;
 
 	/* reset the entry point so if we get another intr we won't