x86: fix cpu-hotplug regression

Commit d435d862
("cpu hotplug: mce: fix cpu hotplug error handling")
changed the error handling in mce_cpu_callback.

In cases where not all CPUs are brought up during
boot (e.g. using maxcpus and additional_cpus parameters)
mce_cpu_callback now returns NOTFIY_BAD because
for such CPUs cpu_data is not completely filled when
the notifier is called. Thus mce_create_device fails right
at its beginning:

        if (!mce_available(&cpu_data[cpu]))
                return -EIO;

As a quick fix I suggest to check boot_cpu_data for MCE.

To reproduce this regression:

(1) boot with maxcpus=2 addtional_cpus=2 on a 4 CPU x86-64 system
(2) # echo 1 >/sys/devices/system/cpu/cpu2/online
  -bash: echo: write error: Invalid argument

dmesg shows:

_cpu_up: attempt to bring up CPU 2 failed
Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/cpu/mcheck/mce_64.c

@@ -810,7 +810,7 @@ static __cpuinit int mce_create_device(unsigned int cpu)
 	int err;
 	int i;
 
-	if (!mce_available(&cpu_data(cpu)))
+	if (!mce_available(&boot_cpu_data))
 		return -EIO;
 
 	memset(&per_cpu(device_mce, cpu).kobj, 0, sizeof(struct kobject));