HID: protect hid_debug_list

Accesses to hid_device->hid_debug_list are not serialized properly, which
could result in SMP concurrency issues when HID debugfs events are accessesed
by multiple userspace processess.

Serialize all the list operations by a mutex.

Spotted by Al Viro.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>

include/linux/hid.h

@@ -512,6 +512,7 @@ struct hid_device {							/* device report descriptor */
 	struct dentry *debug_rdesc;
 	struct dentry *debug_events;
 	struct list_head debug_list;
+	struct mutex debug_list_lock;
 	wait_queue_head_t debug_wait;
 };