pgsql: Don't initialize page in {vm,fsm}_extend(), not needed

Don't initialize page in {vm,fsm}_extend(), not needed

The read path needs to be able to initialize pages anyway, as relation
extensions are not durable. By avoiding initializing pages, we can, in a
future patch, extend the relation by multiple blocks at once.

Using smgrextend() for {vm,fsm}_extend() is not a good idea in general, as at
least one page of the VM/FSM will be read immediately after, always causing a
cache miss, requiring us to read content we just wrote.

Discussion: https://postgr.es/m/20230301223515.pucbj7nb54n4i4nv@awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3d6a98457d8e21d85bed86cfd3e1d1df1b260721

Modified Files
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src/backend/access/heap/visibilitymap.c   | 6 +-----
src/backend/storage/freespace/freespace.c | 5 +----
2 files changed, 2 insertions(+), 9 deletions(-)