kernel - Improve namecache performance

kernel - Improve namecache performance

* Improve performance for the edge case where a process is
  deleting a large number of files.  In this situation,
  NCF_DESTROYED ncp entries can build up in the hash table
  and slow things down until the system quiets down, then
  the tables get cleaned up.

  Improve by recycling such entries into new entries when
  possible.

* Refactor the hash calculation again.  The big-prime idea
  actually has some trivially obvious problems, scrap it
  for the moment.