kernel - Fix SMP tsleep_interlock() vs wakeup() race

kernel - Fix SMP tsleep_interlock() vs wakeup() race

* Problem triggered by kern_mutex.c, probably only triggered by mutexes.
  The mutex sets state non-atomically and calls wakeup() with no
  intervening atomic op.  wakeup() loads the cpumask to wakeup
  non-atomically.  This can cause the wakeup() code to miss an
  interaction with the cpumask set by another cpu.

  cpu A:
	set state			<-- sets state
	wakeup()			<-- loads cpu mask non-atomically

  cpu B:
	tsleep_interlock()		<-- sets cpu mask atomically
	if (checkstate failed)		<-- checks state
		tsleep()

* Fix the problem by add a lfence() in the wakeup() code prior to
  loading the cpumask.  Even though I'm fairly sure only the kern_mutex.c
  code can causing this problem, putting the lfence in wakeup() is the
  safest mechanic.