Fix bogus logic for checking data dirs' versions within pg_upgrade.

Commit 9be95ef1 failed to cure all of the redundancy here: we were
actually calling get_major_server_version() three times for each
of the old and new data directories.  While that's not enormously
expensive, it's still sloppy.

A. Akenteva

Discussion: https://postgr.es/m/f9266a85d918a3cf3a386b5148aee666@postgrespro.ru

src/bin/pg_upgrade/check.c

@@ -234,9 +234,9 @@ check_cluster_versions(void)
 {
 	prep_status("Checking cluster versions");
 
-	/* get old and new cluster versions */
-	old_cluster.major_version = get_major_server_version(&old_cluster);
-	new_cluster.major_version = get_major_server_version(&new_cluster);
+	/* cluster versions should already have been obtained */
+	Assert(old_cluster.major_version != 0);
+	Assert(new_cluster.major_version != 0);
 
 	/*
 	 * We allow upgrades from/to the same major version for alpha/beta

src/bin/pg_upgrade/exec.c

@@ -331,9 +331,8 @@ check_data_dir(ClusterInfo *cluster)
 {
 	const char *pg_data = cluster->pgdata;
 
-	/* get old and new cluster versions */
-	old_cluster.major_version = get_major_server_version(&old_cluster);
-	new_cluster.major_version = get_major_server_version(&new_cluster);
+	/* get the cluster version */
+	cluster->major_version = get_major_server_version(cluster);
 
 	check_single_dir(pg_data, "");
 	check_single_dir(pg_data, "base");