linux-aio: add `dev_max_batch` parameter to laio_io_unplug()

Between the submission of a request and the unplug, other devices
with larger limits may have been queued new requests without flushing
the batch.

Using the new `dev_max_batch` parameter, laio_io_unplug() can check
if the batch exceeds the device limit to flush the current batch.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20211026162346.253081-4-sgarzare@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>