x86: fix early panic with boot option "nosmp"

Impact: fix boot crash with numcpus=0 on certain systems

Fix early exception in __get_smp_config with nosmp.

Bail out early when there is no MP table.
Reported-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NAndi Kleen <ak@linux.intel.com>
Tested-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/mpparse.c

@@ -604,6 +604,9 @@ static void __init __get_smp_config(unsigned int early)
 		printk(KERN_INFO "Using ACPI for processor (LAPIC) "
 		       "configuration information\n");
 
+	if (!mpf)
+		return;
+
 	printk(KERN_INFO "Intel MultiProcessor Specification v1.%d\n",
 	       mpf->mpf_specification);
 #if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86_32)