kernel - Minor pmap optimizations, minor swap_pager*() optimizations

kernel - Minor pmap optimizations, minor swap_pager*() optimizations

* Avoid the atomic_fcmpset_int() in pmap_enter() if the page already
  has the appropriate PG_* bits set.  Because the page is soft-busied,
  this check should not be able to race any clearing of the bits (which
  can only be done when the page is hard-busied).

* swap_pager_freespace*() no longer bothers to acquire the object
  token if the object's swblock_count is 0.

* Misc __read_* optimizations

* Misc __predict* optimizations