kernel - Improve namecache generation handling

kernel - Improve namecache generation handling

* Reduce mount->mnt_namecache_gen from 64 to 32 bits and add a dummy
  field so the structure size does not change.

* Reduce namecache->nc_namecache_gen from 64 to 32 bits and add a
  second generation number to detect cache_unlink() and cache_rename()
  calls.  Bump the counter in cache_rename() and _cache_unlink().
  Structure size did not change.

* Refactor kern_rename() to use namecache->nc_generation to detect
  a larger subset of changes to the namecache entries which can leak
  in due to the temporary unlock of fromnd->nl_nch.ncp.