kernel - vm_page BUSY handling, change vm_page_cache() API, maybe fix seg-fault

kernel - vm_page BUSY handling, change vm_page_cache() API, maybe fix seg-fault

* All vm_page_deactivate() calls now operate with the caller holding
  the page PG_BUSY or the page known not to be busy.  Reorder several
  cases where a vm_page is unbusied prior to calling deactivate.

* vm_page_cache() now expected the vm_page to be PG_BUSY and will cache
  the page and clear the bit.

* Fix a race in vm_pageout_page_free() which calls vm_object_reference()
  with an unbusied vm_page, then proceeds to busy and free the page.
  The problem is that vm_object_reference() can block on vmobj_token.

  This may fix the x86-64 seg-fault issue.  Or it may not (throws up hands).

* Remove incorrect KKASSERT which was causing bogus panics.