VPP-1408: Fix bug that wrong tx fifo drop in buffer chains flow of dgram mode

-- Not drop session dgram hdr when to drop processed udp message from tx fifo

Change-Id: Id5ce5780817041bca05011f7b9b53e97a6d78da4
Signed-off-by: shubing guo <guo.shubing@zte.com.cn>
diff --git a/src/vnet/session/session_node.c b/src/vnet/session/session_node.c
index fcb3a3d..2c425ab 100644
--- a/src/vnet/session/session_node.c
+++ b/src/vnet/session/session_node.c
@@ -307,7 +307,10 @@
 
 	      hdr->data_offset += n_bytes_read;
 	      if (hdr->data_offset == hdr->data_length)
-		svm_fifo_dequeue_drop (f, hdr->data_length);
+		{
+		  u32 offset = hdr->data_length + SESSION_CONN_HDR_LEN;
+		  svm_fifo_dequeue_drop (f, offset);
+		}
 	    }
 	  else
 	    n_bytes_read = svm_fifo_dequeue_nowait (ctx->s->server_tx_fifo,