kernel/cgroup: cleanup cgroup_base_files when fail to add cgroup_psi_files

Even though css_clear_dir would be called to cleanup
all existing cgroup files when css_populate_dir failed,
reclaiming newly created cgroup files before
css_populate_dir returns with failure makes code more
consistent.

Signed-off-by: David Wang <00107082@163.com>
Signed-off-by: Tejun Heo <tj@kernel.org>