pgsql: Fix off-by-one error in read IO tracing

Fix off-by-one error in read IO tracing

AsyncReadBuffer()'s no-IO needed path passed
TRACE_POSTGRESQL_BUFFER_READ_DONE the wrong block number because it had
already incremented operation->nblocks_done. Fix by folding the
nblocks_done offset into the blocknum local variable at initialization.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/u73un3xeljr4fiidzwi4ikcr6vm7oqugn4fo5vqpstjio6anl2%40hph6fvdiiria
Backpatch-through: 18

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cf66978d7990b219a2eda03c3d4f978f68d046d3

Modified Files
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src/backend/storage/buffer/bufmgr.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)