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/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,