x86, microcode: Unregister syscore_ops after microcode unloaded

Currently, microcode doesn't unregister syscore_ops after it's
unloaded. So if we modprobe then rmmod microcode, the stale
microcode syscore_ops info will stay on syscore_ops_list.

Later when we're trying to reboot/halt/shutdown the machine, kernel
will panic on syscore_shutdown().

With the patch applied, I can reboot/halt/shutdown my machine successfully.
Signed-off-by: Xiaotian Feng <dfeng@redhat.com>
Cc: Tigran Aivazian <tigran@aivazian.fsnet.co.uk>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
LKML-Reference: <1301387672-23661-1-git-send-email-dfeng@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/microcode_core.c

@@ -550,6 +550,7 @@ static void __exit microcode_exit(void)
 	microcode_dev_exit();
 
 	unregister_hotcpu_notifier(&mc_cpu_notifier);
+	unregister_syscore_ops(&mc_syscore_ops);
 
 	get_online_cpus();
 	mutex_lock(&microcode_mutex);