kernel - SWAP CACHE part 1/many - Convert swblock to a Red-Black tree

kernel - SWAP CACHE part 1/many - Convert swblock to a Red-Black tree

* Convert struct swblock from being hashed to a per-vm_object RB tree.
  This remove two pointers from struct swblock but adds a RB_ENTRY which
  is three pointers and an integer, so swblock gets a little more
  bloated.

* Optimize swp_pager_meta_free_all().  We previously indexed through
  the entire VM object's size which doesn't scale well for 64-bit
  or for swap-cached vnodes.  Now we need only iterate the RB tree.

* Move swblock fields out of the VM pager union and make them part of the
  native vm_object structure.  Swap block assignments will soon be allowed
  on vnodes for fast data caching.