bpf: use skb_at_tc_ingress helper in tcf_bpf

We have a small skb_at_tc_ingress() helper for testing for ingress, so
make use of it. cls_bpf already uses it and so should act_bpf.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>