core/cgroup: fix return value of unit_cgorup_freezer_action()

We should return 0 only if current freezer state, as reported by the
kernel, is already the desired state. Otherwise, we would dispatch
return dbus message prematurely in bus_unit_method_freezer_generic().

Thanks to Frantisek Sumsal for reporting the issue.

src/core/cgroup.c

@@ -3634,7 +3634,7 @@ int unit_cgroup_freezer_action(Unit *u, FreezerAction action) {
         if (r < 0)
                 return r;
 
-        return 0;
+        return 1;
 }
 
 static const char* const cgroup_device_policy_table[_CGROUP_DEVICE_POLICY_MAX] = {