rocker: Provide offload indication using nexthop flags

We want to stop using the FIB info's flags to provide the offlaod
indication and instead do that on a per-nexthop basis.

Convert rocker to do just that. It only supports one nexthop per-route,
so conversion is simple.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>