io_uring: add comments on why PF_EXITING checking is safe

We have two checks of task->flags & PF_EXITING left:

1) In io_req_task_submit(), which is called in task_work and hence always
   in the context of the original task. That means that
   req->task == current, and hence checking ->flags is totally fine.

2) In io_poll_rewait(), where we need to stop re-arming poll to prevent
   it interfering with cancelation. This is only run from task_work as
   well, and hence for this case too req->task == current.

Add a comment to both spots detailing that.
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2112,6 +2112,7 @@ static void io_req_task_submit(struct io_kiocb *req)
 
 	/* ctx stays valid until unlock, even if we drop all ours ctx->refs */
 	mutex_lock(&ctx->uring_lock);
+	/* req->task == current here, checking PF_EXITING is safe */
 	if (likely(!(req->task->flags & PF_EXITING)))
 		__io_queue_sqe(req);
 	else
@@ -4893,6 +4894,7 @@ static bool io_poll_rewait(struct io_kiocb *req, struct io_poll_iocb *poll)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 
+	/* req->task == current here, checking PF_EXITING is safe */
 	if (unlikely(req->task->flags & PF_EXITING))
 		WRITE_ONCE(poll->canceled, true);