pgsql: Use ExtendBufferedRelTo() in {vm,fsm}_extend()

Use ExtendBufferedRelTo() in {vm,fsm}_extend()

This uses ExtendBufferedRelTo(), introduced in 31966b151e6, to extend the
visibilitymap and freespacemap to the size needed.

It also happens to fix a warning introduced in 3d6a98457d8, reported by Tom
Lane.

Discussion: https://postgr.es/m/20221029025420.eplyow6k7tgu6he3@awork3.anarazel.de
Discussion: https://postgr.es/m/2194723.1680736788@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fcdda1e4b50249c344e510ea93d4bd74d2743430

Modified Files
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src/backend/access/heap/visibilitymap.c   | 80 ++++++++++-------------------
src/backend/storage/freespace/freespace.c | 83 +++++++++----------------------
2 files changed, 48 insertions(+), 115 deletions(-)