cgroup-util: make cg_pid_get_path() return -ENODATA when controller can't be found

If the controller managed by systemd cannot found in /proc/$PID/cgroup,
return ENODATA, the usual error for cases where the data being looked
for does not exist, even if the process does.

src/basic/cgroup-util.c

@@ -870,7 +870,7 @@ int cg_pid_get_path(const char *controller, pid_t pid, char **path) {
                 return 0;
         }
 
-        return -ENOENT;
+        return -ENODATA;
 }
 
 int cg_install_release_agent(const char *controller, const char *agent) {