Bugfix: bitmask in PTE wrong (racecondition)

idea of how the bug worked (in hierarchical pages):
  - currently selected onpagemask is not full.
  - thread starts searching for free slot
  - new free slots appear where the thread already searched, while the
    free slots in unvisited places disappear
  - thread does not find a free slot, sets `_ptes[page].bitmask` a
    position of the onpagemask to 1, indicating that the
    onpagemask/segment is completely filled.
  - since `_ptes[page].bitmask` indicates, that the onpagemask/segment
    is completely filled, no other threads will ever enter this
    segment.
  - The bug hits, since the  `_ptes[page].bitmask` on position
    onpagemask is only set to 0 by a thread that had a completely
    filled onpagemask and removes 1 element. Since the onpagemask was
    never completely filled, no thread will perform the reset
  - memory is lost.