cgroup: use CSS_ONLINE instead of CGRP_DEAD

Use CSS_ONLINE on the self css to indicate whether a cgroup has been
killed instead of CGRP_DEAD.  This will allow re-using css online test
for cgroup liveliness test.  This doesn't introduce any functional
change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>