xen: swiotlb: Open code map_resource callback

General dma_direct_map_resource() is going to be removed
in next patch, so simply open-code it in xen driver.

Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/e9c66a92e818f416875441b6711963f9782dbbeb.1757423202.git.leonro@nvidia.com