virtio_ring: check use_dma_api before unmap desc for indirect

Inside detach_buf_split(), if use_dma_api is false,
vring_unmap_one_split_indirect will be called many times, but actually
nothing is done. So this patch check use_dma_api firstly.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230810123057.43407-2-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>