pgsql: Fix initialization of FDW batching in ExecInitModifyTable

Fix initialization of FDW batching in ExecInitModifyTable

ExecInitModifyTable has to initialize batching for all result relations,
not just the first one. Furthermore, when junk filters were necessary,
the pointer pointed past the mtstate->resultRelInfo array.

Per reports from multiple non-x86 animals (florican, locust, ...).

Discussion: https://postgr.es/m/20200628151002.7x5laxwpgvkyiu3q@development

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/920f853dc948b98a5dc96580c4ee011a302e33e4

Modified Files
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src/backend/executor/nodeModifyTable.c | 31 +++++++++++++++++++++----------
1 file changed, 21 insertions(+), 10 deletions(-)