pktgen: use cpus_read_lock() in pg_net_init()

In the Linux kernel, the following vulnerability has been resolved: pktgen: use cpus_read_lock() in pg_net_init() I have seen the WARN_ON(smp_processor_id() != cpu) firing in pktgen_thread_worker() during tests. We must use cpus_read_lock()/cpus_read_unlock() around the for_each_online_cpu(cpu) loop. While we are at it use WARN_ON_ONCE() to avoid a possible syslog flood.