bpf: btf: allow typedef func_proto

Current implementation does not allow typedef func_proto.
But it is actually allowed.
  -bash-4.4$ cat t.c
  typedef int (f) (int);
  f *g;
  -bash-4.4$ clang -O2 -g -c -target bpf t.c -Xclang -target-feature -Xclang +dwarfris
  -bash-4.4$ pahole -JV t.o
  File t.o:
  [1] PTR (anon) type_id=2
  [2] TYPEDEF f type_id=3
  [3] FUNC_PROTO (anon) return=4 args=(4 (anon))
  [4] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
  -bash-4.4$

This patch related btf verifier to allow such (typedef func_proto)
patterns.

Fixes: 2667a262 ("bpf: btf: Add BTF_KIND_FUNC and BTF_KIND_FUNC_PROTO")
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/btf.c

@@ -1459,7 +1459,8 @@ static int btf_modifier_resolve(struct btf_verifier_env *env,
 
 		/* "typedef void new_void", "const void"...etc */
 		if (!btf_type_is_void(next_type) &&
-		    !btf_type_is_fwd(next_type)) {
+		    !btf_type_is_fwd(next_type) &&
+		    !btf_type_is_func_proto(next_type)) {
 			btf_verifier_log_type(env, v->t, "Invalid type_id");
 			return -EINVAL;
 		}