bindings: make rmdir behave more consistently

bindings: make rmdir behave more consistently

We should be able to safely return -EPERM for:

 	if (!pick_controller_from_path())
                /* Someone's trying to delete "/cgroup". */

 	if (!find_cgroup_in_path())
 	        /* Someone's trying to delete a controller e.g. "/blkio". */

 	if (!get_cgdir_and_path()) {
 		/* Someone's trying to delete a cgroup on the same level as the
 		 * "/lxc" cgroup e.g. rmdir "/cgroup/blkio/lxc" or
 		 * rmdir "/cgroup/blkio/init.slice".
 		 */
 	}

All other interesting cases are caught further down.

Signed-off-by: Christian Brauner <cbrauner@suse.de>