bpftool: Fix the error when lookup in no-btf maps

When reworking btf__get_from_id() in commit a19f93c the error
handling when calling bpf_btf_get_fd_by_id() changed. Before the rework
if bpf_btf_get_fd_by_id() failed the error would not be propagated to
callers of btf__get_from_id(), after the rework it is. This lead to a
change in behavior in print_key_value() that now prints an error when
trying to lookup keys in maps with no btf available.

Fix this by following the way used in dumping maps to allow to look up
keys in no-btf maps, by which it decides whether and where to get the
btf info according to the btf value type.

Fixes: a19f93c ("libbpf: Add internal helper to load BTF data by FD")
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/1644249625-22479-1-git-send-email-yinjun.zhang@corigine.com

tools/bpf/bpftool/map.c

@@ -1054,11 +1054,9 @@ static void print_key_value(struct bpf_map_info *info, void *key,
 	json_writer_t *btf_wtr;
 	struct btf *btf;
 
-	btf = btf__load_from_kernel_by_id(info->btf_id);
-	if (libbpf_get_error(btf)) {
-		p_err("failed to get btf");
+	btf = get_map_kv_btf(info);
+	if (libbpf_get_error(btf))
 		return;
-	}
 
 	if (json_output) {
 		print_entry_json(info, key, value, btf);