asix: fix BUG in receive path when lowering MTU

There is bug in the receive path of the asix driver at the time a
packet is received larger than MTU size and DF bit set:

 BUG: unable to handle kernel paging request at 0000004000000001
 IP: [<ffffffff8126f65b>] skb_release_head_state+0x2d/0xd2
 ...
 Call Trace:
  <IRQ>
  [<ffffffff8126f86d>] ? skb_release_all+0x9/0x1e
  [<ffffffff8126f8ad>] ? __kfree_skb+0x9/0x6f
  [<ffffffffa00b4200>] ? asix_rx_fixup_internal+0xff/0x1ae [asix]
  [<ffffffffa00fb3dc>] ? usbnet_bh+0x4f/0x226 [usbnet]
  ...

It is easily reproducable by setting an MTU of 512 e. g. and sending
something like

  ping -s 1472 -c 1 -M do $SELF

from another box.

And this is because the rx->ax_skb is freed on error, but rx->ax_skb
is not reset, and the size is not reset to zero in this case.

And since the skb is added again to the usbnet->done skb queue it is
accessing already freed memory, resulting in the BUG when freeing a
2nd time.  I therefore think the value 0x0000004000000001 show in the
trace is more or less random data.
Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/asix_common.c

@@ -100,6 +100,9 @@ int asix_rx_fixup_internal(struct usbnet *dev, struct sk_buff *skb,
 			netdev_err(dev->net, "asix_rx_fixup() Bad RX Length %d\n",
 				   rx->size);
 			kfree_skb(rx->ax_skb);
+			rx->ax_skb = NULL;
+			rx->size = 0U;
+
 			return 0;
 		}