libbpf: Honor autocreate flag for struct_ops maps

Skip load steps for struct_ops maps not marked for automatic creation.
This should allow to load bpf object in situations like below:

    SEC("struct_ops/foo") int BPF_PROG(foo) { ... }
    SEC("struct_ops/bar") int BPF_PROG(bar) { ... }

    struct test_ops___v1 {
    	int (*foo)(void);
    };

    struct test_ops___v2 {
    	int (*foo)(void);
    	int (*does_not_exist)(void);
    };

    SEC(".struct_ops.link")
    struct test_ops___v1 map_for_old = {
    	.test_1 = (void *)foo
    };

    SEC(".struct_ops.link")
    struct test_ops___v2 map_for_new = {
    	.test_1 = (void *)foo,
    	.does_not_exist = (void *)bar
    };

Suppose program is loaded on old kernel that does not have definition
for 'does_not_exist' struct_ops member. After this commit it would be
possible to load such object file after the following tweaks:

    bpf_program__set_autoload(skel->progs.bar, false);
    bpf_map__set_autocreate(skel->maps.map_for_new, false);
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20240306104529.6453-4-eddyz87@gmail.com

tools/lib/bpf/libbpf.c

@@ -1212,6 +1212,9 @@ static int bpf_object__init_kern_struct_ops_maps(struct bpf_object *obj)
 		if (!bpf_map__is_struct_ops(map))
 			continue;
 
+		if (!map->autocreate)
+			continue;
+
 		err = bpf_map__init_kern_struct_ops(map);
 		if (err)
 			return err;
@@ -8133,11 +8136,20 @@ static void bpf_map_prepare_vdata(const struct bpf_map *map)
 
 static int bpf_object_prepare_struct_ops(struct bpf_object *obj)
 {
+	struct bpf_map *map;
 	int i;
 
-	for (i = 0; i < obj->nr_maps; i++)
-		if (bpf_map__is_struct_ops(&obj->maps[i]))
-			bpf_map_prepare_vdata(&obj->maps[i]);
+	for (i = 0; i < obj->nr_maps; i++) {
+		map = &obj->maps[i];
+
+		if (!bpf_map__is_struct_ops(map))
+			continue;
+
+		if (!map->autocreate)
+			continue;
+
+		bpf_map_prepare_vdata(map);
+	}
 
 	return 0;
 }