bpf: Resolve modifiers when walking structs

It is impossible to use skb_frag_t in the tracing program. Resolve typedefs
when walking structs.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230626212522.2414485-1-sdf@google.com

kernel/bpf/btf.c

@@ -6135,6 +6135,8 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 
 	*flag = 0;
 again:
+	if (btf_type_is_modifier(t))
+		t = btf_type_skip_modifiers(btf, t->type, NULL);
 	tname = __btf_name_by_offset(btf, t->name_off);
 	if (!btf_type_is_struct(t)) {
 		bpf_log(log, "Type '%s' is not a struct\n", tname);