swiotlb-xen: add missing xen_dma_map_page call

swiotlb-xen is missing a xen_dma_map_page call in
xen_swiotlb_map_sg_attrs, in the bounce buffer path.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>