bpf: task_group_seq_get_next: cleanup the usage of next_thread()

1. find_pid_ns() + get_pid_task() under rcu_read_lock() guarantees that we
   can safely iterate the task->thread_group list. Even if this task exits
   right after get_pid_task() (or goto retry) and pid_alive() returns 0.

   Kill the unnecessary pid_alive() check.

2. next_thread() simply can't return NULL, kill the bogus "if (!next_task)"
   check.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230905154646.GA24928@redhat.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/task_iter.c

@@ -75,15 +75,8 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
 		return NULL;
 
 retry:
-	if (!pid_alive(task)) {
-		put_task_struct(task);
-		return NULL;
-	}
-
 	next_task = next_thread(task);
 	put_task_struct(task);
-	if (!next_task)
-		return NULL;
 
 	saved_tid = *tid;
 	*tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);