bpftool: Show map IDs along with struct_ops links.

A new link type, BPF_LINK_TYPE_STRUCT_OPS, was added to attach
struct_ops to links. (226bc6ae6405) It would be helpful for users to
know which map is associated with the link.

The assumption was that every link is associated with a BPF program, but
this does not hold true for struct_ops. It would be better to display
map_id instead of prog_id for struct_ops links. However, some tools may
rely on the old assumption and need a prog_id.  The discussion on the
mailing list suggests that tools should parse JSON format. We will maintain
the existing JSON format by adding a map_id without removing prog_id. As
for plain text format, we will remove prog_id from the header line and add
a map_id for struct_ops links.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20230421214131.352662-1-kuifeng@meta.com

src/link.c

@@ -212,7 +212,10 @@ static int show_link_close_json(int fd, struct bpf_link_info *info)
 	case BPF_LINK_TYPE_NETFILTER:
 		netfilter_dump_json(info, json_wtr);
 		break;
-
+	case BPF_LINK_TYPE_STRUCT_OPS:
+		jsonw_uint_field(json_wtr, "map_id",
+				 info->struct_ops.map_id);
+		break;
 	default:
 		break;
 	}
@@ -245,7 +248,10 @@ static void show_link_header_plain(struct bpf_link_info *info)
 	else
 		printf("type %u  ", info->type);
 
-	printf("prog %u  ", info->prog_id);
+	if (info->type == BPF_LINK_TYPE_STRUCT_OPS)
+		printf("map %u  ", info->struct_ops.map_id);
+	else
+		printf("prog %u  ", info->prog_id);
 }
 
 static void show_link_attach_type_plain(__u32 attach_type)