io_uring: allow IO_SQE_* flags on IORING_OP_TIMEOUT

There's really no reason why we forbid things like link/drain etc on
regular timeout commands. Enable the usual SQE flags on timeouts.
Reported-by: 李通洲 <carter.li@eoitek.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2703,9 +2703,6 @@ static int io_timeout(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	int ret;
 
 	ret = io_timeout_setup(req);
-	/* common setup allows flags (like links) set, we don't */
-	if (!ret && sqe->flags)
-		ret = -EINVAL;
 	if (ret)
 		return ret;