sit: impement ->ndo_tunnel_ctl

Implement the ->ndo_tunnel_ctl method, and use ip_tunnel_ioctl to
handle userspace requests for the SIOCGETTUNNEL, SIOCADDTUNNEL,
SIOCCHGTUNNEL and SIOCDELTUNNEL ioctls.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>