kernel - Fix MP races in recent vm_object work

kernel - Fix MP races in recent vm_object work

* lwkt_token_swap() needs a critical section to ensure that an interrupt
  does not catch it mid-swap.

* Adjust vm_object_terminate() and vm_object_vnterminate() mechanics to
  require that the object be locked and to eat the lock.

* Use vm_object_hold/drop in vm_map_copy_entry() (object must be locked
  upon call to vm_object_collapse()).

* Acquire object locks in various places and swap the token order as
  needed to chain locks in various loops.

* Shift some code around in vm_object.c