raise_mce() fiddles with global state, but lacks any kind of
serialization.

Add a mutex around the raise_mce() call, so concurrent writers do not
stomp on each other toes.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NChen Gong <gong.chen@linux.intel.com>
Signed-off-by: NTony Luck <tony.luck@intel.com>