MFC some small optimizations:
rev. 1.151: Remove impossible (hk_peer == NULL) check.
rev. 1.152: Remove ng_setisr() call from ng_dequeue().
rev. 1.153: There is no need to erase hook->hk_node before freeing hook.
rev. 1.154: Use new atomic_fetchadd() primitive instead of looping atomic_cmpset().
rev. 1.158: ng_address_hook() microoptimization.