net: lapbether: Close the LAPB device before its underlying Ethernet device closes

When a virtual LAPB device's underlying Ethernet device closes, the LAPB
device is also closed.

However, currently the LAPB device is closed after the Ethernet device
closes. It would be better to close it before the Ethernet device closes.
This would allow the LAPB device to transmit a last frame to notify the
other side that it is disconnecting.

Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>