cgroup: Implement cgroup_file_show()

Add cgroup_file_show() which allows toggling visibility of a cgroup file
using the new kernfs_deactivate(). This will be used to hide psi interface
files on cgroups where it's disabled.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>