pktgen: use min() to make code cleaner

Use min() in order to make code cleaner. Issue found by coccinelle.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: David S. Miller <davem@davemloft.net>