kernel - SWAP CACHE part 7/many - Add vm_swapcache.c core (write side)
* Add vm_swapcache.c which will be responsible for assigning swap to clean
vnode-backed VM pages and writing the data out.
Implement a very simple inactive queue scanner and swap-writer for
testing.
* Track swap space use, split up into the piece used for anonymous
data and the piece used for clean vnode-backed data.
* Add PG_SWAPPED tracking for newly allocated VM pages via
swap_pager_page_inserted().
* Conditionalize the swap code's dirtying/undirtying of VM pages. We
don't want to mess with the dirty state when working the swap
cache since it isn't the definitive backing store for the VM page.