bpf: Allow sk lookup helpers in cgroup skb

Currently sk lookup helpers are allowed in tc, xdp, sk skb, and cgroup
sock_addr programs.

But they would be useful in cgroup skb as well so that for example
cgroup skb ingress program can lookup a peer socket a packet comes from
on same host and make a decision whether to allow or deny this packet
based on the properties of that socket, e.g. cgroup that peer socket
belongs to.

Allow the following sk lookup helpers in cgroup skb:
* bpf_sk_lookup_tcp;
* bpf_sk_lookup_udp;
* bpf_sk_release;
* bpf_skc_lookup_tcp.
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/f8c7ee280f1582b586629436d777b6db00597d63.1589486450.git.rdna@fb.com

net/core/filter.c

@@ -6159,6 +6159,14 @@ cg_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_skb_cgroup_id_proto;
 #endif
 #ifdef CONFIG_INET
+	case BPF_FUNC_sk_lookup_tcp:
+		return &bpf_sk_lookup_tcp_proto;
+	case BPF_FUNC_sk_lookup_udp:
+		return &bpf_sk_lookup_udp_proto;
+	case BPF_FUNC_sk_release:
+		return &bpf_sk_release_proto;
+	case BPF_FUNC_skc_lookup_tcp:
+		return &bpf_skc_lookup_tcp_proto;
 	case BPF_FUNC_tcp_sock:
 		return &bpf_tcp_sock_proto;
 	case BPF_FUNC_get_listener_sock: