Add warning about potentially missing O_DIRECT support or bad alignment

Add warning about potentially missing O_DIRECT support or bad alignment

If the first O_DIRECT fails, then it's likely because:

1) The file system does not support O_DIRECT, or
2) The user set iomem_align to an unsupported value.

Let the user know, otherwise they only see EINVAL and may not realize
why things aren't working.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>