virtio: virtio_dma_buf: fix missing parameter documentation

Add missing parameter documentation for virtio_dma_buf_attach()
function to fix kernel-doc warnings:

Warning: drivers/virtio/virtio_dma_buf.c:41 function parameter 'dma_buf' not described in 'virtio_dma_buf_attach'
Warning: drivers/virtio/virtio_dma_buf.c:41 function parameter 'attach' not described in 'virtio_dma_buf_attach'

The function documentation was missing descriptions for both the
'dma_buf' and 'attach' parameters. Add proper parameter documentation
following kernel-doc format.

Signed-off-by: WangYuli <wangyuli@uniontech.com>
Message-Id: <241C7118259DA110+20250623065210.270237-1-wangyuli@uniontech.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>