kernel - SWAP CACHE part 6/many - Refactor swap_pager_freespace()

kernel - SWAP CACHE part 6/many - Refactor swap_pager_freespace()

* Refactor swap_pager_freespace() to use a RB_SCAN() instead of a
  vm_pindex_t iteration.  This is necessary if we intend to allow
  swap backing store for vnodes because the related files & VM objects
  can be huge.  This is also generally a good idea in 64-bit mode
  to help deal with x86_64's massive address space.

* Start adding swap space freeing calls in the OBJT_VNODE handling code
  and generic VM object handling code.

* Remove various checks for OBJT_SWAP from swap*() and swp*() functions
  to allow them to be used with OBJT_VNODE objects.

* Add checks for degenerate cases to reduce call overheads as the swap
  handling functions are now called for vnode objects too.

* Add assertions for pagers which do not need swap support.