pgsql: Fix buffer usage stats for nodes above Gather Merge.

Fix buffer usage stats for nodes above Gather Merge.

Commit 85c9d347 addressed a similar problem for Gather and Gather
Merge nodes but forgot to account for nodes above parallel nodes.  This
still works for nodes above Gather node because we shut down the workers
for Gather node as soon as there are no more tuples.  We can do a similar
thing for Gather Merge as well but it seems better to account for stats
during nodes shutdown after completing the execution.

Reported-by: Stéphane Lorek, Jehan-Guillaume de Rorthais
Author: Jehan-Guillaume de Rorthais <jgdr@dalibo.com>
Reviewed-by: Amit Kapila
Backpatch-through: 10, where it was introduced
Discussion: https://postgr.es/m/20200718160206.584532a2@firost

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/603c18b7ee2e100f081bb756b54758b38c59f65f

Modified Files
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src/backend/executor/execProcnode.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)