netrom: Fix use-after-free caused by accept on already connected socket

If you call listen() and accept() on an already connect()ed
AF_NETROM socket, accept() can successfully connect.
This is because when the peer socket sends data to sendmsg,
the skb with its own sk stored in the connected socket's
sk->sk_receive_queue is connected, and nr_accept() dequeues
the skb waiting in the sk->sk_receive_queue.

As a result, nr_accept() allocates and returns a sock with
the sk of the parent AF_NETROM socket.

And here use-after-free can happen through complex race conditions:
```
                  cpu0                                                     cpu1
                                                               1. socket_2 = socket(AF_NETROM)
                                                                        .
                                                                        .
                                                                  listen(socket_2)
                                   ...