selftests/bpf: Run cgroup1_hierarchy test in own mount namespace

The cgroup1_hierarchy test uses setup_classid_environment to setup
cgroupv1 environment. The problem is that the environment is set in
/sys/fs/cgroup and therefore, if not run under an own mount namespace,
effectively deletes all system cgroups:

$ ls /sys/fs/cgroup | wc -l
27
$ sudo ./test_progs -t cgroup1_hierarchy
#41/1 cgroup1_hierarchy/test_cgroup1_hierarchy:OK
#41/2 cgroup1_hierarchy/test_root_cgid:OK
#41/3 cgroup1_hierarchy/test_invalid_level:OK
#41/4 cgroup1_hierarchy/test_invalid_cgid:OK
#41/5 cgroup1_hierarchy/test_invalid_hid:OK
#41/6 cgroup1_hierarchy/test_invalid_cgrp_name:OK
#41/7 cgroup1_hierarchy/test_invalid_cgrp_name2:OK
#41/8 cgroup1_hierarchy/test_sleepable_prog:OK
#41 cgroup1_hierarchy:OK
Summary: 1/8 PASSED, 0 SKIPPED, 0 FAILED
$ ls /sys/fs/cgroup | wc -l
1

To avoid this, run setup_cgroup_environment first which will create an
own mount namespace. This only affects the cgroupv1_hierarchy test as
all other cgroup1 test progs already run setup_cgroup_environment prior
to running setup_classid_environment.

Also add a comment to the header of setup_classid_environment to warn
against this invalid usage in future.

Fixes: 360769233cc9 ("selftests/bpf: Add selftests for cgroup1 hierarchy")
Signed-off-by: Viktor Malik <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/cgroup_helpers.c

···
 /**
  * setup_classid_environment() - Setup the cgroupv1 net_cls environment
  *
+ * This function should only be called in a custom mount namespace, e.g.
+ * created by running setup_cgroup_environment.
+ *
  * After calling this function, cleanup_classid_environment should be called
  * once testing is complete.
  *

tools/testing/selftests/bpf/prog_tests/cgroup1_hierarchy.c

···
 		goto destroy;
 
 	/* Setup cgroup1 hierarchy */
+	err = setup_cgroup_environment();
+	if (!ASSERT_OK(err, "setup_cgroup_environment"))
+		goto destroy;
 	err = setup_classid_environment();
 	if (!ASSERT_OK(err, "setup_classid_environment"))
-		goto destroy;
+		goto cleanup_cgroup;
 
 	err = join_classid();
 	if (!ASSERT_OK(err, "join_cgroup1"))
···
 
 cleanup:
 	cleanup_classid_environment();
+cleanup_cgroup:
+	cleanup_cgroup_environment();
 destroy:
 	test_cgroup1_hierarchy__destroy(skel);
 }