kernel - refactor cpusync and pmap_inval code, fix lockup

kernel - refactor cpusync and pmap_inval code, fix lockup

* Simplify the cpusync API.  The API now has only one initialization call,
  one simplified rollup call, and two primary calls handling a single
  function callback (instead of three callbacks).

  cpusync_interlock() interlocks the specified set of cpus and ensures they
  are running in a safe place, cpusync_deinterlock() executes the initialized
  function on the cpu set and does not return until all cpus have completed
  the operation.

* Simplify the pmap_inval per-platform API.  pmap_inval_interlock() and
  pmap_inval_deinterlock() now reflect similar functionality to the cpusync
  API.  pmap/pte operations are now synchronized when pmap_inval_deinterlock()
  is called and not when pmap_inval_done() is called, reducing the range of
  code which can execute while the cpu set is held quiescent.

  pmap_inval_flush() has been removed.  Critical section handling has also
  been rearranged slightly in the pmap_inval* and cpusync* APIs.

* Fixes a cpusync lockup which occurs when the x86-64 pmap and pmap_inval
  code is used to hold a cpu mask quiescent across complex subsystem calls.
  Primarily accomplished by moving the synchronization out of
  pmap_inval_flush() and into pmap_inval_deinterlock().