ip,ip6: introduce basic ip/ip6 loadbalance

Using the configured nexthop GR_NH_T_GROUP, we select one
of the nexthop based on the input RSS.
As of now, we consider all weights being equal to "1".
A further commit will take into account the route weight.

Signed-off-by: Christophe Fontaine <[email protected]>

Author: christophefontaine