Avoid diskIO on master for create index and reindex.

relpages is not updated on master during create index and reindex, as
master need to have agreegated value from segments. As not updating the
relpages, avoid calling RelationGetNumberOfBlocks for master in this
function.

src/backend/catalog/index.c

@@ -1248,7 +1248,6 @@ static void
 index_update_stats(Relation rel, bool hasindex, bool isprimary,
 				   Oid reltoastidxid, double reltuples)
 {
-	BlockNumber relpages = RelationGetNumberOfBlocks(rel);
 	Oid			relid = RelationGetRelid(rel);
 	Relation	pg_class;
 	HeapTuple	tuple;
@@ -1365,6 +1364,9 @@ index_update_stats(Relation rel, bool hasindex, bool isprimary,
 			rd_rel->reltuples = (float4) reltuples;
 			dirty = true;
 		}
+
+		BlockNumber relpages = RelationGetNumberOfBlocks(rel);
+
 		if (rd_rel->relpages != (int32) relpages)
 		{
 			rd_rel->relpages = (int32) relpages;