mlxsw: spectrum_router: Separate nexthop offload indication from route

The driver currently uses the 'RTNH_F_OFFLOAD' flag for both routes and
nexthops, which is cumbersome and unnecessary now that we have separate
flag for the route itself.

Separate the offload indication for nexthops from routes and call it
whenever the offload state within the nexthop group changes.

Note that IPv6 (unlike IPv4) does not share the same nexthop group
between different routes, whereas mlxsw does. Therefore, whenever the
offload indication within an IPv6 nexthop group changes, all the linked
routes need to be updated.

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