swiotlb-xen: ensure we have a single callsite for xen_dma_map_page

Refactor the code a bit to make further changes easier.

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>