UPerf: marshaller adds 24 bytes to estimated size to avoid frequent r…

…esizings of request buffer

tests/perf/org/jgroups/tests/perf/UPerf.java

@@ -456,7 +456,7 @@ public int estimatedSize(Object arg) {
             if(arg == null)
                 return 2;
             if(arg instanceof byte[])
-                return msg_size;
+                return msg_size + 24;
             if(arg instanceof Long)
                 return 10;
             return 50;