pgsql: Fix explicit valgrind interaction in read_stream.c.

Fix explicit valgrind interaction in read_stream.c.

This is a back-patch of commits 2a8a0067 and 2509b857 into
REL_17_STABLE.  It's doesn't fix any known live bug in PostgreSQL v17
itself, but an extension could in theory have used the per-buffer data
feature and seen spurious errors under Valgrind.

Discussion: https://postgr.es/m/CA%2BhUKG%2Bg6aXpi2FEHqeLOzE%2BxYw%3DOV%2B-N5jhOEnnV%2BF0USM9xA%40mail.gmail.com

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/57dca6faa9bdfd3c2ad2805fc088dc768d36fcf0

Modified Files
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src/backend/storage/aio/read_stream.c | 38 +++++++++++++++++++++++++++++------
1 file changed, 32 insertions(+), 6 deletions(-)