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>
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