forcedeth: use per cpu to collect xmit/recv statistics

When testing with a background iperf pushing 1Gbit/sec traffic and running
both ifconfig and netstat to collect statistics, some deadlocks occurred.

Ifconfig and netstat will call nv_get_stats64 to get software xmit/recv
statistics. In the commit f5d827ae ("forcedeth: implement
ndo_get_stats64() API"), the normal tx/rx variables is to collect tx/rx
statistics. The fix is to replace normal tx/rx variables with per
cpu 64-bit variable to collect xmit/recv statistics. The per cpu variable
will avoid deadlocks and provide fast efficient statistics updates.

In nv_probe, the per cpu variable is initialized. In nv_remove, this
per cpu variable is freed.

In xmit/recv process, this per cpu variable will be updated.

In nv_get_stats64, this per cpu variable on each cpu is added up. Then
the driver can get xmit/recv packets statistics.

A test runs for several days with this commit, the deadlocks disappear
and the performance is better.

Tested:
   - iperf SMP x86_64 ->
   Client connecting to 1.1.1.108, TCP port 5001
   TCP window size: 85.0 KByte (default)
   ------------------------------------------------------------
   [  3] local 1.1.1.105 port 38888 connected with 1.1.1.108 port 5001
   [ ID] Interval       Transfer     Bandwidth
   [  3]  0.0-10.0 sec  1.10 GBytes   943 Mbits/sec

   ifconfig results:

   enp0s9 Link encap:Ethernet  HWaddr 00:21:28:6f:de:0f
          inet addr:1.1.1.105  Bcast:0.0.0.0  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:5774764531 errors:0 dropped:0 overruns:0 frame:0
          TX packets:633534193 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:7646159340904 (7.6 TB) TX bytes:11425340407722 (11.4 TB)

   netstat results:

   Kernel Interface table
   Iface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg
   ...
   enp0s9 1500 0  5774764531 0    0 0      633534193      0      0  0 BMRU
   ...

Fixes: f5d827ae ("forcedeth: implement ndo_get_stats64() API")
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>
Signed-off-by: David S. Miller <davem@davemloft.net>