ax25: fix NPD bug in ax25_disconnect

The ax25_disconnect() in ax25_kill_by_device() is not
protected by any locks, thus there is a race condition
between ax25_disconnect() and ax25_destroy_socket().
when ax25->sk is assigned as NULL by ax25_destroy_socket(),
a NULL pointer dereference bug will occur if site (1) or (2)
dereferences ax25->sk.

ax25_kill_by_device()                | ax25_release()
  ax25_disconnect()                  |   ax25_destroy_socket()
    ...                              |
    if(ax25->sk != NULL)             |     ...
      ...                            |     ax25->sk = NULL;
      bh_lock_sock(ax25->sk); //(1)  |     ...
      ...                            |
      bh_unlock_sock(ax25->sk); //(2)|

This patch moves ax25_disconnect() into lock_sock(), which can
synchronize with ax25_destroy_socket() in ax25_release().

Fail log:
===============================================================
BUG: kernel NULL pointer dereference, address: 0000000000000088
...
RIP: 0010:_raw_spin_lock+0x7e/0xd0
...
Call Trace:
ax25_disconnect+0xf6/0x220
ax25_device_event+0x187/0x250
raw_notifier_call_chain+0x5e/0x70
dev_close_many+0x17d/0x230
rollback_registered_many+0x1f1/0x950
unregister_netdevice_queue+0x133/0x200
unregister_netdev+0x13/0x20
...
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ax25/af_ax25.c

@@ -92,8 +92,8 @@ static void ax25_kill_by_device(struct net_device *dev)
 			lock_sock(sk);
 			s->ax25_dev = NULL;
 			ax25_dev_put(ax25_dev);
-			release_sock(sk);
 			ax25_disconnect(s, ENETUNREACH);
+			release_sock(sk);
 			spin_lock_bh(&ax25_list_lock);
 			sock_put(sk);
 			/* The entry could have been deleted from the