[report?] A race condition is suspected.

[Hyunwoo Kim <v4bel@theori.io>]

Dear all,

From the code, it looks like a use-after-free due to a race condition 
could be caused by the lack of a proper lock between the ncsi_dev_work() 
worker and the vlan ioctl:
```
                  cpu0                                          cpu1

         ncsi_dev_work()
           ncsi_configure_channel()
             set_one_vid()
               rcu_read_lock()
                                                            sock_ioctl()
                                                              vlan_ioctl_handler()
                                                                unregister_vlan_dev()
                                                                  vlan_vid_del()
                                                                    __vlan_vid_del()
                                                                      vlan_kill_rx_filter_info()
                                                                        ncsi_vlan_rx_kill_vid()
                                                                          list_del_rcu(&vlan->list);
                                                                          kfree(vlan);
               vid = vlan->vid;    // use-after-free?
               rcu_read_unlock()
```

However, I do not have a device that uses the ncsi_dev_work() worker, 
so I have not been able to debug this.

Can a race condition like this actually happen, and if so, it seems 
like we need to change `kfree(vlan);` to `kfree_rcu(vlan)`, or add 
an appropriate lock to the worker.


Regards,
Hyunwoo Kim