x86/apic: Do apic driver probe for "nosmp" use case

For the "nosmp" use case, the APIC initialization code selects
"APIC_SYMMETRIC_IO_NO_ROUTING" as the default interrupt mode and avoids
probing APIC drivers.

This works well for the default APIC modes, but for the x2APIC case the
probe function is required to allocate the cluster_hotplug mask. So in the
APIC_SYMMETRIC_IO_NO_ROUTING case when the x2APIC is initialized it
dereferences a NULL pointer and the kernel crashes.

This was observed on a TDX platform where x2APIC is enabled and "nosmp"
command line option is allowed.

To fix this issue, probe APIC drivers via default_setup_apic_routing() for
the APIC_SYMMETRIC_IO_NO_ROUTING interrupt mode too.
Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/a64f864e1114bcd63593286aaf61142cfce384ea.1650076869.git.sathyanarayanan.kuppuswamy@intel.com

arch/x86/kernel/apic/apic.c

@@ -1428,22 +1428,21 @@ void __init apic_intr_mode_init(void)
 		return;
 	case APIC_VIRTUAL_WIRE:
 		pr_info("APIC: Switch to virtual wire mode setup\n");
-		default_setup_apic_routing();
 		break;
 	case APIC_VIRTUAL_WIRE_NO_CONFIG:
 		pr_info("APIC: Switch to virtual wire mode setup with no configuration\n");
 		upmode = true;
-		default_setup_apic_routing();
 		break;
 	case APIC_SYMMETRIC_IO:
 		pr_info("APIC: Switch to symmetric I/O mode setup\n");
-		default_setup_apic_routing();
 		break;
 	case APIC_SYMMETRIC_IO_NO_ROUTING:
 		pr_info("APIC: Switch to symmetric I/O mode setup in no SMP routine\n");
 		break;
 	}
 
+	default_setup_apic_routing();
+
 	if (x86_platform.apic_post_init)
 		x86_platform.apic_post_init();