io_uring: Fix leaked shadow_req

io_queue_link_head() owns shadow_req after taking it as an argument.
By not freeing it in case of an error, it can leak the request along
with taken ctx->refs.

Reviewed-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2413,6 +2413,7 @@ static int io_queue_link_head(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	if (ret) {
 		if (ret != -EIOCBQUEUED) {
 			io_free_req(req);
+			__io_free_req(shadow);
 			io_cqring_add_event(ctx, s->sqe->user_data, ret);
 			return 0;
 		}