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>

include/linux/cgroup.h

@@ -143,8 +143,6 @@ static inline void css_put(struct cgroup_subsys_state *css)
 
 /* bits in struct cgroup flags field */
 enum {
-	/* Control Group is dead */
-	CGRP_DEAD,
 	/*
 	 * Control Group has previously had a child cgroup or a task,
 	 * but no longer (only if CGRP_NOTIFY_ON_RELEASE is set)