Use a real RT index when setting up partition tuple routing.

Before, we always used a dummy value of 1, but that's not right when
the partitioned table being modified is inside of a WITH clause
rather than part of the main query.

Amit Langote, reported and reviewd by Etsuro Fujita, with a comment
change by me.

Discussion: http://postgr.es/m/ee12f648-8907-77b5-afc0-2980bcb0aa37@lab.ntt.co.jp

src/backend/executor/execMain.c

@@ -3213,6 +3213,7 @@ EvalPlanQualEnd(EPQState *epqstate)
  */
 void
 ExecSetupPartitionTupleRouting(Relation rel,
+							   Index resultRTindex,
 							   PartitionDispatch **pd,
 							   ResultRelInfo **partitions,
 							   TupleConversionMap ***tup_conv_maps,
@@ -3271,7 +3272,7 @@ ExecSetupPartitionTupleRouting(Relation rel,
 
 		InitResultRelInfo(leaf_part_rri,
 						  partrel,
-						  1,	/* dummy */
+						  resultRTindex,
 						  rel,
 						  0);
 

src/backend/executor/nodeModifyTable.c

@@ -1914,6 +1914,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 					num_partitions;
 
 		ExecSetupPartitionTupleRouting(rel,
+									   node->nominalRelation,
 									   &partition_dispatch_info,
 									   &partitions,
 									   &partition_tupconv_maps,