Fix more holes with SLRU code in need of int64 for segment numbers

Fix more holes with SLRU code in need of int64 for segment numbers

This is a continuation of c9e24573905b, containing changes included into
the proposed patch that have been missed in the actual commit.  I have
managed to miss these diffs while doing a rebase of the original patch.

Thanks to Noah Misch, Peter Eisentraut and Alexander Korotkov for the
pokes.

Discussion: https://postgr.es/m/92fe572d-638e-4162-aef6-1c42a2936f25@eisentraut.org
Discussion: https://postgr.es/m/20240810175055.cd.nmisch@google.com
Backpatch-through: 17