vcl: resolve VCL part session cleanup issue

In Nginx performance test, we notice that there are so many session kept
open with the state of TRANSPORT_DELETE, and it is root caused that
vppcom fails to return close state.

Type: fix

Change-Id: Ia30f959e0e5adb5b8bcfa13fb1dd7b2cb410a31b
Signed-off-by: Yu Ping <ping.yu@intel.com>

src/vcl/vppcom.c

@@ -1733,6 +1733,8 @@ vppcom_session_read_internal (uint32_t session_handle, void *buf, int n,
     {
       if (is_nonblocking)
 	{
+	  if (vcl_session_is_closing (s))
+	    return vcl_session_closing_error (s);
 	  svm_fifo_unset_event (s->rx_fifo);
 	  return VPPCOM_EWOULDBLOCK;
 	}