dummy: Add ndo_uninit().

In register_netdevice(), when ndo_init() is successful and later
some error occurred, ndo_uninit() will be called.
So dummy deivce is desirable to implement ndo_uninit() method
to free percpu stats for this case.
And, ndo_uninit() is also called along with dev->destructor() when
device is unregistered, so in order to prevent dev->dstats from
being freed twice, dev->destructor is modified to free_netdev().

Signed-off-by: Hiroaki SHIMODA <shimoda.hiroaki@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>