taskstats: add_del_listener() should ignore !valid listeners

When send_cpu_listeners() finds the orphaned listener it marks it as
!valid and drops listeners->sem. Before it takes this sem for writing,
s->pid can be reused and add_del_listener() can wrongly try to re-use
this entry.

Change add_del_listener() to check ->valid = T.

Signed-off-by: Oleg Nesterov <[e-mail]>
Reviewed-by: Vasiliy Kulikov <[e-mail]>
Acked-by: Balbir Singh <[e-mail]>
Cc: Jerome Marchand <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/taskstats.c

···
 			listeners = &per_cpu(listener_array, cpu);
 			down_write(&listeners->sem);
 			list_for_each_entry(s2, &listeners->list, list) {
-				if (s2->pid == pid)
+				if (s2->pid == pid && s2->valid)
 					goto exists;
 			}
 			list_add(&s->list, &listeners->list);