pgsql: Change LogicalTapeSetBlocks() to use nBlocksWritten.

Change LogicalTapeSetBlocks() to use nBlocksWritten.

Previously, it was based on nBlocksAllocated to account for tapes with
open write buffers that may not have made it to the BufFile yet.

That was unnecessary, because callers do not need to get the number of
blocks while a tape has an open write buffer; and it also conflicted
with the preallocation logic added for HashAgg.

Reviewed-by: Peter Geoghegan
Discussion: https://postgr.es/m/ce5af05900fdbd0e9185747825a7423c48501964.camel@j-davis.com
Backpatch-through: 13

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6e146a663536f86c8421ac6ed08c4eb9a69979fd

Modified Files
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src/backend/executor/nodeAgg.c   |  2 +-
src/backend/utils/sort/logtape.c | 12 +++++++++++-
2 files changed, 12 insertions(+), 2 deletions(-)