compio's flush is not error safe

[Xuanwo]

compio has similiar issues like tokio-rs/tokio#6325, but in the other way:

compio/compio-io/src/write/buf.rs

Lines 100 to 107 in 4cabfec

	async fn flush(&mut self) -> IoResult<()> {
	let Self { writer, buf } = self;
	buf.with(|w| writer.write_all(w)).await?;
	buf.reset();
	Ok(())
	}

While error happened during write, compio could write more data than expected.