io_uring: fortify tctx/io_wq cleanup

We don't want anyone poking into tctx->io_wq awhile it's being destroyed
by io_wq_put_and_exit(), and even though it shouldn't even happen, if
buggy would be preferable to get a NULL-deref instead of subtle delayed
failure or UAF.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/827b021de17926fd807610b3e53a5a5fa8530856.1621513214.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -9035,15 +9035,15 @@ static void io_uring_del_task_file(unsigned long index)
 
 static void io_uring_clean_tctx(struct io_uring_task *tctx)
 {
+	struct io_wq *wq = tctx->io_wq;
 	struct io_tctx_node *node;
 	unsigned long index;
 
+	tctx->io_wq = NULL;
 	xa_for_each(&tctx->xa, index, node)
 		io_uring_del_task_file(index);
-	if (tctx->io_wq) {
-		io_wq_put_and_exit(tctx->io_wq);
-		tctx->io_wq = NULL;
-	}
+	if (wq)
+		io_wq_put_and_exit(wq);
 }
 
 static s64 tctx_inflight(struct io_uring_task *tctx, bool tracked)