io_uring: pin ctx on fallback execution

Pin ring in io_fallback_req_func() by briefly elevating ctx->refs in
case any task_work handler touches ctx after releasing a request.

Fixes: 9011bf9 ("io_uring: fix stuck fallback reqs")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/833a494713d235ec144284a9bbfe418df4f6b61c.1629235576.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2477,8 +2477,10 @@ static void io_fallback_req_func(struct work_struct *work)
 	struct llist_node *node = llist_del_all(&ctx->fallback_llist);
 	struct io_kiocb *req, *tmp;
 
+	percpu_ref_get(&ctx->refs);
 	llist_for_each_entry_safe(req, tmp, node, io_task_work.fallback_node)
 		req->io_task_work.func(req);
+	percpu_ref_put(&ctx->refs);
 }
 
 static void __io_complete_rw(struct io_kiocb *req, long res, long res2,