Fix futures_io::AsyncSeek implementaion for Compat

[dhruv9vats]

Motivation

Aims to address #5764.

Solution

Waiting for a seek operation to complete by calling poll_complete before calling start_seek.

[dhruv9vats]

Adding a dependency to add a test feels overkill.
But the thing is, the test only fails for a tokio::fs::file::File.
Using something like tokio::io::util::buf_writer::BufWriter or std::io::cursor::Cursor seems to be working fine on latest upstream/master:

#[tokio::test]
async fn compat_seek() -> futures_util::io::Result<()> {
    let mut cursor = Cursor::new(Vec::new()).compat_write();

    cursor.write_all(&[0, 1, 2, 3, 4, 5]).await?;
    cursor.write_all(&[6, 7]).await?;

    assert_eq!(cursor.stream_position().await?, 8);

    // Modify elements at position 2.
    assert_eq!(cursor.seek(SeekFrom::Start(2)).await?, 2);
    cursor.write_all(&[8, 9]).await?;

    cursor.flush().await?;

    // Verify we still have 8 elements
    assert_eq!(cursor.seek(SeekFrom::End(0)).await?, 8);
    // Seek back to the start to read and verify contents.
    cursor.seek(SeekFrom::Start(0)).await?;

    let mut buf = Vec::new();
    let num_bytes = cursor.read_to_end(&mut buf).await?;
    assert_eq!(&buf[..num_bytes], &[0, 1, 8, 9, 4, 5, 6, 7]);

    Ok(())
}

[dhruv9vats]

@Darksonn, is there some alternative way to test this? And would it more sense to skip adding one if there is none?

[Darksonn]

This dependency already exists in the main Tokio crate, so adding it here doesn't actually add a new dependency.

[Darksonn]

Thanks.