kernel - Fix memory ordering race

kernel - Fix memory ordering race

* Fix a race in the mtx wait/wakeup code for situations where the
  releasing thread hands lock ownership to the waiter.  In this
  situation the waiter can sometimes succeed without having to do
  additional atomic ops.  However, this also allows speculative reads
  by the waiting cpu to preceed the lock handover.

* Add an mfence to fix this problem.  Add a few cpu_sfence()s (which
  are basically NOPs on Intel) to clarify other bits of code too.