net: sched: Add helpers to identify classids

Offloading drivers need to understand what qdisc class a filter is added
to. Currently they only need to identify ingress, clsact->ingress and
clsact->egress. So provide these helpers.

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