x86, microcode: Fix sysfs warning during module unload on unsupported CPUs

commit a956bd6 upstream.

Loading the microcode driver on an unsupported CPU and subsequently
unloading the driver causes

 WARNING: at fs/sysfs/group.c:138 mc_device_remove+0x5f/0x70 [microcode]()
 Hardware name: 01972NG
 sysfs group ffffffffa00013d0 not found for kobject 'cpu0'
 Modules linked in: snd_hda_codec_hdmi snd_hda_codec_conexant snd_hda_intel btusb snd_hda_codec bluetooth thinkpad_acpi rfkill microcode(-) [last unloaded: cfg80211]
 Pid: 4560, comm: modprobe Not tainted 3.4.0-rc2-00002-g258f742 raspberrypi#5
 Call Trace:
  [<ffffffff8103113b>] ? warn_slowpath_common+0x7b/0xc0
  [<ffffffff81031235>] ? warn_slowpath_fmt+0x45/0x50
  [<ffffffff81120e74>] ? sysfs_remove_group+0x34/0x120
  [<ffffffffa00000ef>] ? mc_device_remove+0x5f/0x70 [microcode]
  [<ffffffff81331eb9>] ? subsys_interface_unregister+0x69/0xa0
  [<ffffffff81563526>] ? mutex_lock+0x16/0x40
  [<ffffffffa0000c3e>] ? microcode_exit+0x50/0x92 [microcode]
  [<ffffffff8107051d>] ? sys_delete_module+0x16d/0x260
  [<ffffffff810a0065>] ? wait_iff_congested+0x45/0x110
  [<ffffffff815656af>] ? page_fault+0x1f/0x30
  [<ffffffff81565ba2>] ? system_call_fastpath+0x16/0x1b

on recent kernels.

This is due to commit 8a25a2f ("cpu: convert 'cpu' and
'machinecheck' sysdev_class to a regular subsystem") which renders
commit 6c53cbf ("x86, microcode: Correct sysdev_add error path")
useless.

See http://marc.info/?l=linux-kernel&m=133416246406478

Avoid above warning by restoring the old driver behaviour before
6c53cbf ("x86, microcode: Correct sysdev_add error path").

Cc: Tigran Aivazian <[email protected]>
Signed-off-by: Andreas Herrmann <[email protected]>
Acked-by: Greg Kroah-Hartman <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Borislav Petkov <[email protected]>
[bwh: Backported to 3.2: deleted line uses sys_dev, not dev]
Signed-off-by: Ben Hutchings <[email protected]>

Author: Andreas Herrmann

arch/x86/kernel/microcode_core.c

@@ -418,10 +418,8 @@ static int mc_sysdev_add(struct sys_device *sys_dev)
 	if (err)
 		return err;
 
-	if (microcode_init_cpu(cpu) == UCODE_ERROR) {
-		sysfs_remove_group(&sys_dev->kobj, &mc_attr_group);
+	if (microcode_init_cpu(cpu) == UCODE_ERROR)
 		return -EINVAL;
-	}
 
 	return err;
 }