kernel - Fix panic related to kqueue-based timers

kernel - Fix panic related to kqueue-based timers

* Fix a panic related to kqueue-based timers.  When a knote is destroyed it
  is possible for the callout_stop() to race a callout on another cpu that
  is blocked on a lock.

* Introduce callout_terminate() which stops a callout and ensures that no
  function calls for the callout is still in progress before returning.
  The kevent code now uses this function instead of callout_stop().

* We can't have this assurance for the normal callout_stop() function because
  it can deadlock a backend stuck waiting for a lock when a frontend holding
  that lock calls callout_stop().

Reported-by: Antonio Huete Jimenez <tuxillo@quantumachine.net>