kernel - Fix bugs and refactor namecache cleaning code

kernel - Fix bugs and refactor namecache cleaning code

* Refactor the cleaning code.  For positive namecache entries,
  track based on NCHHASH linkages rather than v_namecache linkages.
  Also maintain a count of freeable (leaf) entries and use that
  in the _cache_cleanpos() code.

  This should hopefully fix a bug where the system can get stuck
  constantly calling _cache_cleanpos() in situations where there
  are not any freeable entries.

* Refactor the negative namecache tracking code.  Move to a per-cpu
  structure where entries are made based on the current cpu and removed
  based on the recorded cpu.  Refactor the cleaning code to iterate
  the cpu list on a per-call basis which should hopefully allow multiple
  cpus to clean the ncneg lists concurrently.

  This reduces a SMP bottleneck, but does not deal with cache
  ping-ponging issues on related structures.

Reported-by: ftigeot