bpf: Simplify check in btf_parse_hdr()

Replace offsetof(hdr_len) + sizeof(hdr_len) with offsetofend(hdr_len) to
simplify the check for correctness of btf_data_size in btf_parse_hdr()
Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220320075240.1001728-1-ytcoode@gmail.com

kernel/bpf/btf.c

@@ -4482,8 +4482,7 @@ static int btf_parse_hdr(struct btf_verifier_env *env)
 	btf = env->btf;
 	btf_data_size = btf->data_size;
 
-	if (btf_data_size <
-	    offsetof(struct btf_header, hdr_len) + sizeof(hdr->hdr_len)) {
+	if (btf_data_size < offsetofend(struct btf_header, hdr_len)) {
 		btf_verifier_log(env, "hdr_len not found");
 		return -EINVAL;
 	}