kernel - Add required vm_token back in to swaponvp()

kernel - Add required vm_token back in to swaponvp()

* This case is not normally hit unless you try to add new swap space
  while the machine is already heavily paging to or from swap.

* The blist operations and vm_swap_size variable manipulation needs
  the token.

* Clean-up the exit code by creating a common exit code section.