net: sched: introduce helper to identify gact trap action

Introduce a helper called is_tcf_gact_trap which could be used to
tell if the action is gact trap or not.

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