freespace: Don't modify page without any lock

freespace: Don't modify page without any lock

Before this commit fsm_vacuum_page() modified the page without any lock on the
page. Historically that was kind of ok, as we didn't rely on the freespace to
really stay consistent and we did not have checksums. But these days pages are
checksummed and there are ways for FSM pages to be included in WAL records,
even if the FSM itself is still not WAL logged. If a FSM page ever were
modified while a WAL record referenced that page, we'd be in trouble, as the
WAL CRC could end up getting corrupted.

The reason to address this right now is a series of patches with the goal to
only allow modifications of pages with an appropriate lock level. Obviously
not having any lock is not appropriate :)

Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/4wggb7purufpto6x35fd2kwhasehnzfdy3zdcu47qryubs2hdz@fa5kannykekr
Discussion: https://postgr.es/m/e6a8f734-2198-4958-a028-aba863d4a204@iki.fi