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

Branch
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master

Details
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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(-)