HAMMER VFS - Hack cursor iterator when unlocked cursor moved to parent

HAMMER VFS - Hack cursor iterator when unlocked cursor moved to parent

* It is possible to reverse-index a cursor while it is unlocked due to
  a node deletion moving cursors on that node to the parent, and a
  subsequent insertion then inserting new elements between the cursor's
  current position and its expected iteration range.

* Detect the case with a new flag (hack!) HAMMER_CURSOR_ITERATE_CHECK
  and just iterate past the elements outside the iteration range in
  this case.

Reported-by: YONETANI Tomokazu <qhwt+dfly@les.ath.cx>