bpftool: Wrap struct_ops dump in an array

When dumping a struct_ops, 2 dictionaries are emitted.

When using `name`, they were already wrapped in an array, but not when
using `id`. Causing `jq` to fail at parsing the payload as it reached
the comma following the first dict.

This change wraps those dictionaries in an array so valid json is emitted.

Before, jq fails to parse the output:
```
$ sudo bpftool struct_ops dump id 1523612 | jq . > /dev/null
parse error: Expected value before ',' at line 19, column 2
```

After, no error parsing the output:
```
sudo ./bpftool struct_ops dump id 1523612 | jq . > /dev/null
```

Signed-off-by: Manu Bretelle <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Tested-by: Eduard Zingerman <[e-mail]>
Acked-by: Eduard Zingerman <[e-mail]>
Acked-by: Quentin Monnet <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/struct_ops.c

···
 
 	res.nr_maps++;
 
+	if (wtr)
+		jsonw_start_array(wtr);
+
 	if (func(fd, info, data, wtr))
 		res.nr_errs++;
 	else if (!wtr && json_output)
···
 		 * other cmds.
 		 */
 		jsonw_null(json_wtr);
+
+	if (wtr)
+		jsonw_end_array(wtr);
 
 done:
 	free(info);