Small patch for GiST: move childoffnum to child

During preparing patch of my GSoC project I found reasonable to move childoffnum (GISTInsertStack structure) from parent to child. This means that now child have childoffnum of parent's link to child. It allows to maintain entire parts of tree in that GISTInsertStack structures. Also it simplifies existing code a bit.

Heikki advice me that since this change simplifies existing code it can be considered as a separate patch.