kernel - Add callback API for mpipe

kernel - Add callback API for mpipe

* Add a callback API for mpipe which uses a dedicated kthread,
  allowing clients to avoid deadlocks related to held locks during
  strategy calls.

* Add mpipe_alloc_callback().  Use of this function also requires
  that MPF_CALLBACK be supplied to mpipe_init().

* Add mpipe_wait().  This function may be used for clients which
  which to roll their own mpipe retry loop (or already have their
  own thread(s) to deal with it in a safe manner).