pgsql: Fix parallel vacuum I/O timing reporting.

Fix parallel vacuum I/O timing reporting.

Commit 5cd72cc0c5 made the VACUUM VERBOSE and autovacuum logs report
buffer usage from the pgBufferUsage delta, which includes the parallel
workers' usage, but left the I/O timings on the leader's
pgStatBlockReadTime and pgStatBlockWriteTime. So the log counted the
blocks the workers read and dirtied but not the time they spent on
them.

Fix this by taking the I/O timings from the same BufferUsage delta.
This also removes a small error in the leader's own timings, since
pgStatBlock{Read,Write}Time accumulate whole microseconds per I/O.

Although 5cd72cc0c5 was backpatched to v15, BufferUsage on v16 doesn't
track the I/O time of temporary relations, so the same fix there would
drop that time from the log. We don't think a reporting issue is worth
v16-specific code, nor worth leaving v16 unfixed between v15 and v17,
so backpatch to v17 only.

Found by Bharath using AI assisted review with Claude.

Reported-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Author: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Reviewed-by: Sami Imseih <samimseih(dot)pg(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Reviewed-by: Shihao Zhong <zhong950419(at)gmail(dot)com>
Discussion: https://postgr.es/m/CALj2ACXrqaHrYmGdpAoGKXtiY9Xw4o1dxSC+EELxn47GDO6HAQ@mail.gmail.com
Backpatch-through: 17

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/35c508af520963bf1245b86f437c21f834cc2be0

Modified Files
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src/backend/access/heap/vacuumlazy.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)