pgsql: Update FSM after updating VM on-access

Update FSM after updating VM on-access

b46e1e54d078de allowed setting the VM while on-access pruning, but it
neglected to update the freespace map. Once the page was all-visible,
vacuum could skip it, leading to stale freespace map values and,
effectively, bloat. Fix it by updating the FSM if we updated the VM.

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Discussion: https://postgr.es/m/flat/CAAKRu_b2StZrEC%3DHmW8LePuQbczyFRnfs8qTAJwn_%3DW76-y24w%40mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e9eaeb04248a8b2cc977440caa1b72174c108c14

Modified Files
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src/backend/access/heap/pruneheap.c | 26 +++++++++++++++++++++++---
1 file changed, 23 insertions(+), 3 deletions(-)