bpf: bpf_iter_task_next: use __next_thread() rather than next_thread()

Lockless use of next_thread() should be avoided, kernel/bpf/task_iter.c
is the last user and the usage is wrong.

bpf_iter_task_next() can loop forever, "kit->pos == kit->task" can never
happen if kit->pos execs. Change this code to use __next_thread().

With or without this change the usage of kit->pos/task and next_task()
doesn't look nice, see the next patch.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20231114163237.GA897@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/task_iter.c

@@ -1015,12 +1015,11 @@ __bpf_kfunc struct task_struct *bpf_iter_task_next(struct bpf_iter_task *it)
 	if (flags == BPF_TASK_ITER_ALL_PROCS)
 		goto get_next_task;
 
-	kit->pos = next_thread(kit->pos);
-	if (kit->pos == kit->task) {
-		if (flags == BPF_TASK_ITER_PROC_THREADS) {
-			kit->pos = NULL;
+	kit->pos = __next_thread(kit->pos);
+	if (!kit->pos) {
+		if (flags == BPF_TASK_ITER_PROC_THREADS)
 			return pos;
-		}
+		kit->pos = kit->task;
 	} else
 		return pos;