Add uid and cookie bpf helper to cg_skb_func_proto

BPF helper functions get_socket_cookie and get_socket_uid can be
used for network traffic classifications, among others. Expose
them also to programs of type BPF_PROG_TYPE_CGROUP_SKB. As of
commit 8f917bba ("bpf: pass sk to helper functions") the
required skb->sk function is available at both cgroup bpf ingress
and egress hooks. With these two new helper, cg_skb_func_proto is
effectively the same as sk_filter_func_proto.

Change since V1:
Instead of add the helper to cg_skb_func_proto, redirect the
cg_skb_func_proto to sk_filter_func_proto since all helper function
in sk_filter_func_proto are applicable to cg_skb_func_proto now.

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