net/ipv4: ip_vti clear skb cb before tunneling.

If users apply shaper to vti tunnel then it will cause a kernel crash. The
problem seems to be due to the vti_tunnel_xmit function not clearing
skb->opt field before passing the packet to xfrm tunneling code.

Signed-off-by: Saurabh Mohan <saurabh@vyatta.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>