x86: use zalloc_cpumask_var for mce_dev_initialized

We need a cleared cpu_mask to record if mce is initialized, especially
when MAXSMP is used.

used zalloc_... instead

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Reviewed-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: stable@kernel.org
Signed-off-by: H. Peter Anvin <hpa@zytor.com>