ALSA: timer: Add missing mutex lock for compat ioctls

ALSA: timer: Add missing mutex lock for compat ioctls

commit 79fb0518fec8c8b4ea7f1729f54f293724b3dbb0 upstream.

The races among ioctl and other operations were protected by the
commit af368027a49a ("ALSA: timer: Fix race among timer ioctls") and
later fixes, but one code path was forgotten in the scenario: the
32bit compat ioctl.  As syzkaller recently spotted, a very similar
use-after-free may happen with the combination of compat ioctls.

The fix is simply to apply the same ioctl_lock to the compat_ioctl
callback, too.

Fixes: af368027a49a ("ALSA: timer: Fix race among timer ioctls")
Reference: http://lkml.kernel.org/r/089e082686ac9b482e055c832617@google.com
Reported-by: syzbot <bot+e5f3c9783e7048a74233054febbe9f1bdf54b6da@syzkaller.appspotmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>