netdev: netdev_priv() can now be sane again.

The private area of a netdev is now at a fixed offset once more.

Unfortunately, some assumptions that netdev_priv() == netdev->priv
crept back into the tree.  In particular this happened in the
loopback driver.  Make it use netdev->ml_priv.

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