{pktgen, xfrm} Construct skb dst for tunnel mode transformation

IPsec tunnel mode encapuslation needs to set outter ip header
with right protocol/ttl/id value with regard to skb->dst->child.

Looking up a rt in a standard way is absolutely wrong for every
packet transmission. In a simple way, construct a dst by setting
neccessary information to make tunnel mode encapuslation working.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>