bond: Get the rtnl_link_ops support correct

- Don't call rtnl_link_unregister if rtnl_link_register fails
- Set .priv_size so we aren't stomping on uninitialized memory
  when we use netdev_priv, on bond devices created with
  ip link add type bond.

Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>