cgroup: Cleanup function usage

cgroup: Cleanup function usage

Some masks shouldn't be needed externally, so keep their functions in
the module (others would fit there too but they're used in tests) to
think twice if something would depend on them.

Drop unused function cg_attach_many_everywhere.

Use cgroup_realized instead of cgroup_path when we actually ask for
realized.

This should not cause any functional changes.