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.

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With best regards,
Alexander Korotkov.