set tx_queuelen to 0 when create veth device

VETH network device will create only one Qdisc queue default, this will become peformance bottleneck.Set tx_queuelen to 0 when create VETH device, kernel will not create Qdisc queue for VETH device, so this will improve network performance.

gperf outputs are as follows:

Samples: 1M of event 'cycles', Event count (approx.): 237661920980
-  13.97%  [kernel]             [k] _spin_lock                     
   - _spin_lock                                                    
      - 37.64% dev_queue_xmit                                      
         + 74.94% br_dev_queue_push_xmit                           
         + 25.06% neigh_resolve_output                             
      + 19.41% try_to_wake_up                                      
      - 17.95% sch_direct_xmit                                     
         - 74.23% __qdisc_run                                      
            - 97.47% dev_queue_xmit                                
               + 84.43% br_dev_queue_push_xmit                     
               + 15.57% neigh_resolve_output                       
            + 2.53% net_tx_action                                  
         - 25.77% dev_queue_xmit                                   
            + 68.81% br_dev_queue_push_xmit                        
            + 31.19% neigh_resolve_output                          
      + 17.53% task_rq_lock                                        
      + 3.71% mod_timer                                            
      + 0.86% enqueue_to_backlog                                   
+   3.82%  [kernel]             [k] update_curr  
+   1.95%  netserver            [.] 0x000000000001dd31             
+   1.86%  [kernel]             [k] update_cfs_shares              
+   1.81%  [kernel]             [k] tcp_ack                        
+   1.76%  [kernel]             [k] schedule                       
+   1.74%  [kernel]             [k] _spin_lock_irq                 
+   1.67%  [kernel]             [k] dev_queue_xmit

As we can see, spin lock consume a lot of CPU. When set tx_queuelen to 0, the result are as follows:

Samples: 3M of event 'cycles', Event count (approx.): 464264275376
-   8.23%  [kernel]              [k] _spin_lock                       
   - _spin_lock                                                       
      - 54.43% dev_queue_xmit                                         
           br_dev_queue_push_xmit                                     
           br_forward_finish                                          
           __br_forward                                               
           br_forward                                                 
           br_handle_frame_finish                                     
           br_handle_frame                                            
           __netif_receive_skb                                        
           process_backlog                                            
           net_rx_action                                              
           __do_softirq                                               
           call_softirq                                               
      - 25.78% sch_direct_xmit                                        
         - 86.24% __qdisc_run                                         
            + 95.66% dev_queue_xmit                                   
            + 4.34% net_tx_action                                     
         + 13.76% dev_queue_xmit                                      
      + 7.12% task_rq_lock                                            
      + 3.56% try_to_wake_up                                          
      + 2.29% mod_timer                                               
      + 2.04% ipt_do_table                                            
      + 1.57% enqueue_to_backlog                                      
      + 0.60% tcp_v4_rcv                                              
+   2.28%  netserver             [.] 0x000000000001a0ca               
+   2.13%  [kernel]              [k] tcp_ack                          
+   2.00%  [kernel]              [k] update_curr                      
+   1.68%  [igb]                 [k] igb_poll 

CPU consumption decreased spin locks.I test with netserver in TCP_RR mode,and performance upgrade from 46W+ to 70W+.

Signed-off-by: Ye Yin  <hustcat@gmail.com>