pgsql: amcheck: Fix parent key check in gin_index_check()

amcheck: Fix parent key check in gin_index_check()

The checks introduced by commit 14ffaece0fb5 did not get the parent key
checks quite right, missing some data corruption cases. In particular:

* The "rightlink" check was not working as intended, because rightlink
  is a BlockNumber, and InvalidBlockNumber is 0xFFFFFFFF, so

    !GinPageGetOpaque(page)->rightlink

  almost always evaluates to false (except for rightlink=0). So in most
  cases parenttup was left NULL, preventing any checks against parent.

* Use GinGetDownlink() to retrieve child blkno to avoid triggering
  Assert, same as the core GIN code.

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

Author: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Reviewed-by: Andrey M. Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/CAE7r3MJ611B9TE=YqBBncewp7-k64VWs+sjk7XF6fJUX77uFBA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cdd1a431f21dbd2b7b675a9db1c24b97d713f38b

Modified Files
--------------
contrib/amcheck/t/006_verify_gin.pl | 78 +++++++++++++++++++++++++++++++++++++
contrib/amcheck/verify_gin.c        |  8 ++--
2 files changed, 82 insertions(+), 4 deletions(-)