ipvs: Better derivation of ipvs in ip_vs_tunnel_xmit

Don't use "net_ipvs(skb_net(skb))" as skb_net is a bad hack.  Instead
use cp->ipvs and ipvs->net for the net.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>