sit: set rtnl_link_ops before calling register_netdevice

When creating a SIT tunnel with ip tunnel, rtnl_link_ops is not set before
ipip6_tunnel_create is called. When register_netdevice is called, there is
no linkinfo attribute in the NEWLINK message because of that.

Setting rtnl_link_ops before calling register_netdevice fixes that.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>