Update call_rcu() usage, add synchronize_rcu()

      all the readers who were traversing the list when we deleted the

      element are finished.  We use <function>call_rcu()</function> to

      register a callback which will actually destroy the object once

      the readers are finished.

      all pre-existing readers are finished.  Alternatively,

      <function>synchronize_rcu()</function> may be used to block until

      all pre-existing are finished.

    </para>

    <para>

      But how does Read Copy Update know when the readers are

-        object_put(obj);

+        list_del_rcu(&obj->list);

         cache_num--;

+        call_rcu(&obj->rcu, cache_delete_rcu, obj);

+        call_rcu(&obj->rcu, cache_delete_rcu);

 }

 /* Must be holding cache_lock */