selftests/cgroup: fix child process escaping to parent cleanup in test_cpucg_nice

selftests/cgroup: fix child process escaping to parent cleanup in test_cpucg_nice

In test_cpucg_nice, the forked child process incorrectly jumps to the
parent's cleanup label on cg_write failure. This causes the child to
attempt cg_destroy on cgroups the parent is still using, and then
return to main() to continue executing tests as if it were the parent.

Replace goto cleanup with exit(EXIT_FAILURE) in the child process.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>