net: forcedeth: add xmit_more support

This change adds support for xmit_more based on the igb commit 6f19e12f
("igb: flush when in xmit_more mode and under descriptor pressure") and
commit 6b16f9ee ("net: move skb->xmit_more hint to softnet data") that
were made to igb to support this feature. The function netif_xmit_stopped
is called to check whether transmit queue on device is currently unable to
send to determine whether we must write the tail because we can add no
further buffers.

When normal packets and/or xmit_more packets fill up tx_desc, it is
necessary to trigger NIC tx reg.

Following the advice from David Miller and Jakub Kicinski, after the
xmit_more feature is added, the following scenario will occur.

         |
   xmit_more packets
         |
   DMA_MAPPING
         |
   DMA_MAPPING error check
         |
   xmit_more packets already in HW xmit queue
         |

In the above scenario, if DMA_MAPPING error occurrs, the xmit_more packets
already in HW xmit queue will also be dropped. This is different from the
behavior before xmit_more feature. So it is necessary to trigger NIC HW tx
reg in the above scenario.

To the non-xmit_more packets, the above scenario will not occur.

Tested:
  - pktgen (xmit_more packets) SMP x86_64 ->
    Test command:
    ./pktgen_sample03_burst_single_flow.sh ... -b 8 -n 1000000
    Test results:
    Params:
    ...
    burst: 8
    ...
    Result: OK: 12194004(c12188996+d5007) usec, 1000001 (1500byte,0frags)
    82007pps 984Mb/sec (984084000bps) errors: 0

  - iperf (normal packets) SMP x86_64 ->
    Test command:
    Server: iperf -s
    Client: iperf -c serverip
    Result:
    TCP window size: 85.0 KByte (default)
    ------------------------------------------------------------
    [ ID] Interval       Transfer     Bandwidth
    [  3]  0.0-10.0 sec  1.10 GBytes   942 Mbits/sec

CC: Joe Jin <joe.jin@oracle.com>
CC: JUNXIAO_BI <junxiao.bi@oracle.com>
Reported-and-tested-by: Nan san <nan.1986san@gmail.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>