tc, clsact: add clsact frontend

tc, clsact: add clsact frontend

Add the tc part for the kernel commit 1f211a1b929c ("net, sched: add
clsact qdisc"). Quoting example usage from that commit description:

  Example, adding qdisc:

  # tc qdisc add dev foo clsact
  # tc qdisc show dev foo
  qdisc mq 0: root
  qdisc pfifo_fast 0: parent :1 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
  qdisc pfifo_fast 0: parent :2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
  qdisc pfifo_fast 0: parent :3 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
  qdisc pfifo_fast 0: parent :4 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
  qdisc clsact ffff: parent ffff:fff1

  Adding filters (deleting, etc works analogous by specifying ingress/egress):

  # tc filter add dev foo ingress bpf da obj bar.o sec ingress
  # tc filter add dev foo egress  bpf da obj bar.o sec egress
  # tc filter show dev foo ingress
  filter protocol all pref 49152 bpf
  filter protocol all pref 49152 bpf handle 0x1 bar.o:[ingress] direct-action
  # tc filter show dev foo egress
  filter protocol all pref 49152 bpf
  filter protocol all pref 49152 bpf handle 0x1 bar.o:[egress] direct-action

The ingress parent alias can also be used with ingress qdisc.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>