IMPALA-9248: fix test_krpc_rpcz on older kernels

Switched the test to check a different counter that
is present in the 2.6.32 kernel (see
https://kernel.googlesource.com/pub/scm/linux/kernel/git/vapier/lemote/+/linux-2.6.32-stable/include/linux/tcp.h)

Testing:
Ran the affected test on a CentOS 6 system.

Change-Id: I40aece9d37622710a4ef3add2b1088bb3ac91563
Reviewed-on: http://gerrit.cloudera.org:8080/14913
Reviewed-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
diff --git a/tests/webserver/test_web_pages.py b/tests/webserver/test_web_pages.py
index d6f80d3..b264fbe 100644
--- a/tests/webserver/test_web_pages.py
+++ b/tests/webserver/test_web_pages.py
@@ -569,7 +569,11 @@
       assert conn["remote_ip"] != ""
       assert conn["num_calls_in_flight"] >= 0
       assert conn["num_calls_in_flight"] == len(conn["calls_in_flight"])
-      assert conn["socket_stats"]["bytes_acked"] > 0, conn
+      # Check rtt, which should be present in 'struct tcp_info' even in old kernels
+      # like 2.6.32.
+      assert conn["socket_stats"]["rtt"] > 0, conn
+      # send_queue_bytes uses TIOCOUTQ, which is also present in 2.6.32 and even older
+      # kernels.
       assert conn["socket_stats"]["send_queue_bytes"] >= 0, conn
 
   @pytest.mark.execute_serially