mcheck_cmn_handler() does not always set 'severity_cpu' to override
its value taken from previous rounds of MC handling, which will
interfere the current round of MC handling. Always re-initialize it to
clear the historical value.
Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com> Reviewed-by: Jan Beulich <jbeulich@suse.com>