io_uring: flush completions for fallbacks

io_fallback_req_func() doesn't expect anyone creating inline
completions, and no one currently does that. Teach the function to flush
completions preparing for further changes.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8b941516921f72e1a64d58932d671736892d7fff.1629286357.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1197,6 +1197,11 @@ static void io_fallback_req_func(struct work_struct *work)
 	percpu_ref_get(&ctx->refs);
 	llist_for_each_entry_safe(req, tmp, node, io_task_work.fallback_node)
 		req->io_task_work.func(req);
+
+	mutex_lock(&ctx->uring_lock);
+	if (ctx->submit_state.compl_nr)
+		io_submit_flush_completions(ctx);
+	mutex_unlock(&ctx->uring_lock);
 	percpu_ref_put(&ctx->refs);
 }