utests_system_keyspace_directory - more than 500 tests failing on trunk

patch by Berenguer Blasi; reviewed by Sam Tunnicliffe for CASSANDRA-17137

src/java/org/apache/cassandra/db/ColumnFamilyStore.java

@@ -2941,7 +2941,7 @@ public PerDiskFlushExecutors(int flushWriters,
         private static ExecutorPlus[] createPerDiskFlushWriters(int numberOfExecutors, int flushWriters)
         {
             ExecutorPlus[] flushExecutors = new ExecutorPlus[numberOfExecutors];
-            for (int i = 0; i < DatabaseDescriptor.getAllDataFileLocations().length; i++)
+            for (int i = 0; i < numberOfExecutors; i++)
             {
                 flushExecutors[i] = newThreadPool("PerDiskMemtableFlushWriter_"+i, flushWriters);
             }