kernel - Close small race in kqueue

kernel - Close small race in kqueue

* Normalize the kn_status reprocessing code to close a race.
  A knote_detach_and_drop() during a descriptor close can set
  KN_DELETING and return, leaving a knote undergoing processing
  by another thread marked deleted.

  If the other thread is trying to execute a filter the filter can
  wind up being run after the file pointer has been detached
  from the knote, resulting in a panic.

* All KN_DELETING and KN_WAITING flags also now set KN_REPROCESS and
  all KN_REPROCESS handling is dealt with the same way.  The filter
  is run if KN_REPROCESS is not set and if it is set any KN_DELETING
  and KN_WAITING flags are dealt with before the filter is run.