examples/kni: improve zeroing statistics

examples/kni: improve zeroing statistics

The worker threads incrementing the rx/tx_packets race with the signal
handler from the main thread zeroing the entire statistics structure.
This can cause the statistics to fail to be zeroed, even when there
is no traffic on those interfaces.

Improve zeroing the statistics by only incrementing rx/tx_packets
in worker threads by a non-zero amount.  This limits the race to the
periods in which traffic is actually being received or transmitted.

Signed-off-by: Dan Gora <dg@adax.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>