bpf: add btf func and func_proto kind support

bpf: add btf func and func_proto kind support

The issue is discovered for bpf selftest test_skb_cgroup.sh.
Currently we have,
  $ ./test_skb_cgroup_id.sh
  Wait for testing link-local IP to become available ... OK
  Object has unknown BTF type: 13!
  [PASS]

In the above the BTF type 13 refers to BTF kind
BTF_KIND_FUNC_PROTO.
This patch added support of BTF_KIND_FUNC_PROTO and
BTF_KIND_FUNC during type parsing.
With this patch, I got
  $ ./test_skb_cgroup_id.sh
  Wait for testing link-local IP to become available ... OK
  [PASS]

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>