pgsql: Fix amcheck child check pg_upgrade bug.

Fix amcheck child check pg_upgrade bug.

Commit d114cc53 overlooked the fact that pg_upgrade'd B-Tree indexes
have leaf page high keys whose offset numbers do not match the one from
the copy of the tuple one level up (the copy stored with a downlink for
leaf page's right sibling page). This led to false positive reports of
corruption from bt_index_parent_check() when it was called to verify a
pg_upgrade'd index.

To fix, skip comparing the offset number on pg_upgrade'd B-Tree indexes.

Author: Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>
Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Reported-By: Andrew Bille <andrewbille(at)gmail(dot)com>
Diagnosed-By: Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>
Bug: #16619
Discussion: https://postgr.es/m/16619-aaba10f83fdc1c3c@postgresql.org
Backpatch: 13-, where child check was enhanced.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aac80bfcddf7df3fbd1eb73e8a386115d922c714

Modified Files
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contrib/amcheck/verify_nbtree.c | 34 ++++++++++++++++++++++++++++------
1 file changed, 28 insertions(+), 6 deletions(-)