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.