bpf: Use thread_group_leader()

Instead of directly comparing task->tgid and task->pid, use the
thread_group_leader() helper.  This helps with readability, and
there should be no functional change.
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201218185032.2464558-3-jonathan.lemon@gmail.com

kernel/bpf/task_iter.c

@@ -37,7 +37,7 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
 		if (!task) {
 			++*tid;
 			goto retry;
-		} else if (skip_if_dup_files && task->tgid != task->pid &&
+		} else if (skip_if_dup_files && !thread_group_leader(task) &&
 			   task->files == task->group_leader->files) {
 			put_task_struct(task);
 			task = NULL;