kernel - Fix bug in lwkt_token_swap()

kernel - Fix bug in lwkt_token_swap()

* Fix a bug where lwkt_token_swap() would incorrectly assign the ref
  pointer when the top two tokens on the stack are the same token.

  When swapping the top two tokens where tok1 == tok2, the token
  can wind up being repointed to the top ref instead of the deeper
  ref.  When the top ref is released this caused the token to also
  release even though there was another deeper ref to the same
  token.

* Effected the vm_object traversal code and could cause an object token
  to be effectively lost without triggering a panic, resulting in
  corruption.