pgsql: amcheck: Distinguish interrupted page deletion from corruption.

amcheck: Distinguish interrupted page deletion from corruption.

This prevents false-positive reports about "the first child of leftmost
target page is not leftmost of its level", "block %u is not leftmost"
and "left link/right link pair".  They appeared if amcheck ran before
VACUUM cleaned things, after a cluster exited recovery between the
first-stage and second-stage WAL records of a deletion.  Back-patch to
v11 (all supported versions).

Reviewed by Peter Geoghegan.

Discussion: https://postgr.es/m/20231005025232.c7.nmisch@google.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3a205c91665fddf76e333844884354874dc8ae1b

Modified Files
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contrib/amcheck/t/005_pitr.pl   | 88 +++++++++++++++++++++++++++++++++++++++++
contrib/amcheck/verify_nbtree.c | 76 +++++++++++++++++++++++++++++++++--
2 files changed, 160 insertions(+), 4 deletions(-)