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