fixed bug with wrong bitmask in PTE

This bugfix is backported from https://github.com/ComputationalRadiationPhysics/mallocMC/commit/48afb4936e6010048cfd782bcc5f526fe985e3a4

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` at 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.