kernel - Add vm_page_need_commit() and vm_page_clear_commit()

kernel - Add vm_page_need_commit() and vm_page_clear_commit()

* Add vm_page_need_commit() and vm_page_clear_commit().  This controls the
  PG_NEED_COMMIT flag on individual VM pages.

  When set a clean VM page cannot be reused and is still subject to
  flushing by the pageout daemon.

* Refactor buwrite() to use the new mechanism instead of dirtying the
  pages underlying the buffer.

* This allows the normal page and buffer flushing mechanism to operate
  even in situations where the VM page is also the nominal backing store
  for the data (as in tmpfs).

* Will be used by tmpfs.