vdpa/mlx5: Delete direct MKEYs in parallel

Use the async interface to issue MTT MKEY deletion.

This makes destroy_user_mr() on average 8x times faster.
This number is also dependent on the size of the MR being
deleted.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20240830105838.2666587-4-dtatulea@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>