x86/mcheck: Be prepared for a rollback back to the ONLINE state

If we try a CPU down and fail in the middle then we roll back to the
online state. This means we would perform CPU_ONLINE / mce_device_create()
without invoking CPU_DEAD / mce_device_remove() for the cleanup of what was
allocated in CPU_ONLINE.

Be prepared for this and don't allocate the struct if we have it
already.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: rt@linutronix.de
Cc: linux-edac@vger.kernel.org
Link: http://lkml.kernel.org/r/20161110174447.11848-4-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

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

@@ -2409,6 +2409,10 @@ static int mce_device_create(unsigned int cpu)
 	if (!mce_available(&boot_cpu_data))
 		return -EIO;
 
+	dev = per_cpu(mce_device, cpu);
+	if (dev)
+		return 0;
+
 	dev = kzalloc(sizeof *dev, GFP_KERNEL);
 	if (!dev)
 		return -ENOMEM;

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

@@ -1097,6 +1097,10 @@ static int threshold_create_device(unsigned int cpu)
 	struct threshold_bank **bp;
 	int err = 0;
 
+	bp = per_cpu(threshold_banks, cpu);
+	if (bp)
+		return 0;
+
 	bp = kzalloc(sizeof(struct threshold_bank *) * mca_cfg.banks,
 		     GFP_KERNEL);
 	if (!bp)