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