kernel - Cleanup vfs_lock & ref-count states states (2)

* Adjust trigger points such that under normal operation vnlru_proc()
  handles cleaning up extra vnodes.  If this is not sufficient then
  the synchronous cleanup code will kick in at higher levels.

* Adjust vnode->v_act handling and try to take into account vnodes
  with large memory objects (which we would rather reclaim later and
  not sooner).  This takes over functionality from vlru_reclaim().

* Remove the vlrureclaim() mount-scanning infrastructure.  vnlru_proc()
  now just calls freesomevnodes().  This should now be sufficient.  This
  removes significant locking overheads during steady-state operation.
Sort SEE ALSO in some manual pages.