cgroup: make cgroup hold onto its kernfs_node

cgroup currently releases its kernfs_node when it gets removed.  While
not buggy, this makes cgroup->kn access rules complicated than
necessary and leads to things like get/put protection around
kernfs_remove() in cgroup_destroy_locked().  In addition, we want to
use kernfs_name/path() and friends but also want to be able to
determine a cgroup's name between removal and release.

This patch makes cgroup hold onto its kernfs_node until freed so that
cgroup->kn is always accessible.

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