[netdevice] Stop link block timer when device is closed

[netdevice] Stop link block timer when device is closed

A running link block timer holds a reference to the network device and
will prevent it from being freed until the timer expires.  It is
impossible for free_netdev() to be called while the timer is still
running: the call to stop_timer() therein is therefore a no-op.

Stop the link block timer when the device is closed, to allow a
link-blocked device to be freed immediately upon unregistration of the
device.  (Since link block state is updated in response to received
packets, the state is effectively undefined for a closed device: there
is therefore no reason to leave the timer running.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>