mlxsw: spectrum_router: Refresh offload indication upon group refresh

Now that we provide offload indication using the nexthop's flags we must
refresh the offload indication whenever the offload state within the
group changes.

This didn't matter until now, as offload indication was provided using
the FIB info flags and multipath routes were marked as offloaded as long
as one of the nexthops was offloaded.

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