kernel - wakeup() lfence -> mfence

kernel - wakeup() lfence -> mfence

* wakeup() needs to use a mfence() before loading from the global cpumasks
  array.   lfence() is not sufficient.

* Issue arises where the mutex code sets a variable non-atomically and
  then calls wakeup().  If the write to the variable is reordered around
  wakeup()'s read of the global cpumask, then wakeup() can race the mutex
  link wait setup and cause the mutex to stall.