cgroup: use cgroup_kn_lock_live() in other cgroup kernfs methods

Make __cgroup_procs_write() and cgroup_release_agent_write() use
cgroup_kn_lock_live() and cgroup_kn_unlock() instead of
cgroup_lock_live_group().  This puts the operations under both
cgroup_tree_mutex and cgroup_mutex protection without circular
dependency from kernfs active protection.  Also, this means that
cgroup_mutex is no longer nested below kernfs active protection.
There is no longer any place where the two locks interact.

This leaves cgroup_lock_live_group() without any user.  Removed.

This will help simplifying cgroup locking.

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