kthread: update comment for __to_kthread

With commit 343f4c49f243 ("kthread: Don't allocate kthread_struct for init
and umh") and commit 753550eb0ce1 ("fork: Explicitly set PF_KTHREAD"), umh
task no longer have struct kthread and PF_KTHREAD flag.

Update the comment to describe what the current rules are to detect
is something is a kthread.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jiazi Li <[e-mail]>
Signed-off-by: mingzhu.wang <[e-mail]>
Suggested-by Eric W . Biederman <[e-mail]>
Reviewed-by: "Eric W. Biederman" <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

kernel/kthread.c

···
 /*
  * Variant of to_kthread() that doesn't assume @p is a kthread.
  *
- * Per construction; when:
+ * When "(p->flags & PF_KTHREAD)" is set the task is a kthread and will
+ * always remain a kthread.  For kthreads p->worker_private always
+ * points to a struct kthread.  For tasks that are not kthreads
+ * p->worker_private is used to point to other things.
  *
- *   (p->flags & PF_KTHREAD) && p->worker_private
- *
- * the task is both a kthread and struct kthread is persistent. However
- * PF_KTHREAD on it's own is not, kernel_thread() can exec() (See umh.c and
- * begin_new_exec()).
+ * Return NULL for any task that is not a kthread.
  */
 static inline struct kthread *__to_kthread(struct task_struct *p)
 {