net: sched: gred: add basic Qdisc offload

Add basic offload for the GRED Qdisc.  Inform the drivers any
time Qdisc or virtual queue configuration changes.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>