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.