pktgen: make sure that pktgen_thread_worker has been executed

The following courruption can happen during pktgen stop:
list_del corruption. prev->next should be ffff81007e8a5e70, but was 6b6b6b6b6b6b6b6b
kernel BUG at lib/list_debug.c:67!
:pktgen:pktgen_thread_worker+0x374/0x10b0
? autoremove_wake_function+0x0/0x40
? _spin_unlock_irqrestore+0x42/0x80
? :pktgen:pktgen_thread_worker+0x0/0x10b0
kthread+0x4d/0x80
child_rip+0xa/0x12
? restore_args+0x0/0x30
? kthread+0x0/0x80
? child_rip+0x0/0x12
RIP list_del+0x48/0x70

The problem is that pktgen_thread_worker can not be executed if kthread_stop
has been called too early. Insert a completion on the normal initialization
path to make sure that pktgen_thread_worker will gain the control for sure.

Signed-off-by: Denis V. Lunev <[e-mail]>
Acked-by: Alexey Dobriyan <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/pktgen.c

···
 	int cpu;
 
 	wait_queue_head_t queue;
+	struct completion start_done;
 };
 
 #define REMOVE 1
···
 	BUG_ON(smp_processor_id() != cpu);
 
 	init_waitqueue_head(&t->queue);
+	complete(&t->start_done);
 
 	pr_debug("pktgen: starting pktgen/%d:  pid=%d\n", cpu, task_pid_nr(current));
 
···
 	INIT_LIST_HEAD(&t->if_list);
 
 	list_add_tail(&t->th_list, &pktgen_threads);
+	init_completion(&t->start_done);
 
 	p = kthread_create(pktgen_thread_worker, t, "kpktgend_%d", cpu);
 	if (IS_ERR(p)) {
···
 	}
 
 	wake_up_process(p);
+	wait_for_completion(&t->start_done);
 
 	return 0;
 }