bpf: btf: Fix end boundary calculation for type section

The end boundary math for type section is incorrect in
btf_check_all_metas().  It just happens that hdr->type_off
is always 0 for now because there are only two sections
(type and string) and string section must be at the end (ensured
in btf_parse_str_sec).

However, type_off may not be 0 if a new section would be added later.
This patch fixes it.

Fixes: f80442a4cd18 ("bpf: btf: Change how section is supported in btf_header")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/btf.c

@@ -1844,7 +1844,7 @@ static int btf_check_all_metas(struct btf_verifier_env *env)
 
 	hdr = &btf->hdr;
 	cur = btf->nohdr_data + hdr->type_off;
-	end = btf->nohdr_data + hdr->type_len;
+	end = cur + hdr->type_len;
 
 	env->log_type_id = 1;
 	while (cur < end) {