net: sched: consolidate offload decision in cls_u32

The offload decision was originally very basic and tied to if the dev
implemented the appropriate ndo op hook. The next step is to allow
the user to more flexibly define if any paticular rule should be
offloaded or not. In order to have this logic in one function lift
the current check into a helper routine tc_should_offload().

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>