kernel - Rewrite vnode ref-counting code to improve performance

kernel - Rewrite vnode ref-counting code to improve performance

* Rewrite the vnode ref-counting code and modify operation to not
  immediately VOP_INACTIVE a vnode when its refs drops to 0.  By
  doing so we avoid cycling vnodes through exclusive locks when
  temporarily accessing them (such as in a path lookup).  Shared
  locks can be used throughout.

* Track active/inactive vnodes a bit differently, keep track of
  the number of vnodes that are still active but have zero refs,
  and rewrite the vnode freeing code to use the new statistics
  to deactivate cached vnodes.