veth: prevent oops caused by netdev destructor

From: Stephen Hemminger <shemminger@vyatta.com>

The veth driver will oops if sysfs hooks are open while module is removed.

The net device destructor can not point to code in a module; basically
there are only two possible safe values: NULL - no destructor, or
free_netdev - free on last use

Signed-off-by: David S. Miller <davem@davemloft.net>