app/testpmd: fix statistics after reset

app/testpmd: fix statistics after reset

Currently, when running start/clear stats&xstats/stop command many times
based on testpmd application, there are incorrect forward Rx/Tx-packets
stats as below:
---------------------- Forward statistics for port 0  --------------
RX-packets: 18446744073709544808 RX-dropped: 0                <snip>
TX-packets: 18446744073709536616 TX-dropped: 0                <snip>
--------------------------------------------------------------------

The root cause as below:
1. The struct rte_port of testpmd.h has a member variable "struct
   rte_eth_stats stats" to store the last port statistics.
2. When running start command, it execute cmd_start_parsed ->
   start_packet_forwarding -> fwd_stats_reset, which call
   rte_eth_stats_get API function to save current port statistics.
3. When running stop command, it execute fwd_stats_display, which call
   rte_eth_stats_get to get current port statistics, and then minus last
   port statistics.
4. If we run clear stats or xstats after start command, then run stop,
   it may display above incorrect stats because the current
   Rx/Tx-packets is lower than the last saved RX/TX-packets(uint64_t
   overflow).

This patch fixes it by clearing last port statistics when executing
"clear stats/xstats" command.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>