xen/mce: Register native mce handler as vMCE bounce back point

When Xen hypervisor inject vMCE to guest, use native mce handler
to handle it

Signed-off-by: Ke, Liping <[e-mail]>
Signed-off-by: Jiang, Yunhong <[e-mail]>
Signed-off-by: Jeremy Fitzhardinge <[e-mail]>
Signed-off-by: Liu, Jinsong <[e-mail]>
Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>

arch/x86/xen/enlighten.c

···
 	/*
 	 * Look for known traps using IST, and substitute them
 	 * appropriately.  The debugger ones are the only ones we care
-	 * about.  Xen will handle faults like double_fault and
-	 * machine_check, so we should never see them.  Warn if
+	 * about.  Xen will handle faults like double_fault,
+	 * so we should never see them.  Warn if
 	 * there's an unexpected IST-using fault handler.
 	 */
 	if (addr == (unsigned long)debug)
···
 		return 0;
 #ifdef CONFIG_X86_MCE
 	} else if (addr == (unsigned long)machine_check) {
-		return 0;
+		/*
+		 * when xen hypervisor inject vMCE to guest,
+		 * use native mce handler to handle it
+		 */
+		;
 #endif
 	} else {
 		/* Some other trap using IST? */