hamradio: improve the incomplete fix to avoid NPD

The previous commit 3e0588c291d6 ("hamradio: defer ax25 kfree after
unregister_netdev") reorder the kfree operations and unregister_netdev
operation to prevent UAF.

This commit improves the previous one by also deferring the nullify of
the ax->tty pointer. Otherwise, a NULL pointer dereference bug occurs.
Partial of the stack trace is shown below.

BUG: kernel NULL pointer dereference, address: 0000000000000538
RIP: 0010:ax_xmit+0x1f9/0x400
...
Call Trace:
 dev_hard_start_xmit+0xec/0x320
 sch_direct_xmit+0xea/0x240
 __qdisc_run+0x166/0x5c0
 __dev_queue_xmit+0x2c7/0xaf0
 ax25_std_establish_data_link+0x59/0x60
 ax25_connect+0x3a0/0x500
 ? security_socket_connect+0x2b/0x40
 __sys_connect+0x96/0xc0
 ? __hrtimer_init+0xc0/0xc0
 ? common_nsleep+0x2e/0x50
 ? switch_fpu_return+0x139/0x1a0
 __x64_sys_connect+0x11/0x20
 do_syscall_64+0x33/0x40
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The crash point is shown as below

static void ax_encaps(...) {
  ...
  set_bit(TTY_DO_WRITE_WAKEUP, &ax->tty->flags); // ax->tty = NULL!
  ...
}

By placing the nullify action after the unregister_netdev, the ax->tty
pointer won't be assigned as NULL net_device framework layer is well
synchronized.

Signed-off-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hamradio/mkiss.c

@@ -794,14 +794,14 @@ static void mkiss_close(struct tty_struct *tty)
 	 */
 	netif_stop_queue(ax->dev);
 
-	ax->tty = NULL;
-
 	unregister_netdev(ax->dev);
 
 	/* Free all AX25 frame buffers after unreg. */
 	kfree(ax->rbuff);
 	kfree(ax->xbuff);
 
+	ax->tty = NULL;
+
 	free_netdev(ax->dev);
 }