Merge kern_descrip.c:1.284 from HEAD to RELENG_6:

Merge kern_descrip.c:1.284 from HEAD to RELENG_6:

  In closef(), remove the assumption that there is a thread associated
  with the file descriptor.  When a file descriptor is closed as a result
  of garbage collecting a UNIX domain socket, the file descriptor will
  not have any associated thread, so the logic to identify advisory locks
  held by that thread is not appropriate.  Check the thread for NULL to
  avoid this scenario.  Expand an existing comment to say a bit more about
  this.