taskstats: add_del_listener() shouldn't use the wrong node

1. Commit 26c4caea9d69 "don't allow duplicate entries in listener mode"
changed add_del_listener(REGISTER) so that "next_cpu:" can reuse the
listener allocated for the previous cpu, this doesn't look exactly
right even if minor.

Change the code to kfree() in the already-registered case, this case
is unlikely anyway so the extra kmalloc_node() shouldn't hurt but
looke more correct and clean.

2. use the plain list_for_each_entry() instead of _safe() to scan
listeners->list.

3. Remove the unneeded INIT_LIST_HEAD(&s->list), we are going to
list_add(&s->list).

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

kernel/taskstats.c

···
 	if (!cpumask_subset(mask, cpu_possible_mask))
 		return -EINVAL;
 
-	s = NULL;
 	if (isadd == REGISTER) {
 		for_each_cpu(cpu, mask) {
-			if (!s)
-				s = kmalloc_node(sizeof(struct listener),
-						 GFP_KERNEL, cpu_to_node(cpu));
+			s = kmalloc_node(sizeof(struct listener),
+					GFP_KERNEL, cpu_to_node(cpu));
 			if (!s)
 				goto cleanup;
+
 			s->pid = pid;
-			INIT_LIST_HEAD(&s->list);
 			s->valid = 1;
 
 			listeners = &per_cpu(listener_array, cpu);
 			down_write(&listeners->sem);
-			list_for_each_entry_safe(s2, tmp, &listeners->list, list) {
+			list_for_each_entry(s2, &listeners->list, list) {
 				if (s2->pid == pid)
-					goto next_cpu;
+					goto exists;
 			}
 			list_add(&s->list, &listeners->list);
 			s = NULL;
-next_cpu:
+exists:
 			up_write(&listeners->sem);
+			kfree(s); /* nop if NULL */
 		}
-		kfree(s);
 		return 0;
 	}