selftests/cgroup: Fix error path leaks in test_percpu_basic

selftests/cgroup: Fix error path leaks in test_percpu_basic

When cg_name_indexed() returns NULL partway through the child creation
loop, the code returned -1 without running cleanup_children and cleanup.
That left the `parent` pathname allocation unreleased and did not remove
child cgroup directories already created under the parent. Fix by jumping
to cleanup_children instead of returning.

When cg_create() fails, `child` (the pathname from cg_name_indexed())
was not freed before cleanup_children. Fix by freeing `child` before
branching to cleanup_children.

Fixes: 90631e1dea55 ("kselftests: cgroup: add perpcu memory accounting test")
Signed-off-by: Yu Miao <yumiao@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>