libbpf: Set btf_value_type_id of struct bpf_map for struct_ops.

For a struct_ops map, btf_value_type_id is the type ID of it's struct
type. This value is required by bpftool to generate skeleton including
pointers of shadow types. The code generator gets the type ID from
bpf_map__btf_value_type_id() in order to get the type information of the
struct type of a map.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240229064523.2091270-2-thinker.li@gmail.com

Author: ThinkerYzu1

tools/lib/bpf/libbpf.c

@@ -1229,6 +1229,7 @@ static int init_struct_ops_maps(struct bpf_object *obj, const char *sec_name,
 		map->name = strdup(var_name);
 		if (!map->name)
 			return -ENOMEM;
+		map->btf_value_type_id = type_id;
 
 		map->def.type = BPF_MAP_TYPE_STRUCT_OPS;
 		map->def.key_size = sizeof(int);
@@ -4857,6 +4858,10 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
 		create_attr.btf_value_type_id = 0;
 		map->btf_key_type_id = 0;
 		map->btf_value_type_id = 0;
+		break;
+	case BPF_MAP_TYPE_STRUCT_OPS:
+		create_attr.btf_value_type_id = 0;
+		break;
 	default:
 		break;
 	}