amcheck: Fix posting tree checks in gin_index_check()

amcheck: Fix posting tree checks in gin_index_check()

Fix two issues in parent_key validation in posting trees:

* It's not enough to check stack->parentblk is valid to determine if the
  parentkey is valid. It's possible parentblk is set to a valid block
  number, but parentkey is invalid. So check parentkey directly.

* We don't need to invalidate parentkey for all child pages of the
  rightmost page. It's enough to invalidate it for the rightmost child
  only, which means we can check more cases (less false negatives).

Issues reported by Arseniy Mukhin, along with a proposed patch. Review
by Andrey M. Borodin, cleanup and improvements by me.

Author: Arseniy Mukhin <[email protected]>
Reviewed-by: Andrey M. Borodin <[email protected]>
Discussion: https://postgr.es/m/CAE7r3MJ611B9TE=YqBBncewp7-k64VWs+sjk7XF6fJUX77uFBA@mail.gmail.com