swiotlb-xen: introduce phys_to_dma/dma_to_phys translations

With some devices physical addresses are different than dma addresses.
To be able to deal with these cases, we need to call phys_to_dma on
physical addresses (including machine addresses in Xen terminology)
before returning them from xen_swiotlb_alloc_coherent and
xen_swiotlb_map_page.

We also need to convert dma addresses back to physical addresses using
dma_to_phys in xen_swiotlb_free_coherent and xen_swiotlb_unmap_page if
we want to do any operations on them.

Call dma_to_phys in is_xen_swiotlb_buffer.
Introduce xen_phys_to_dma and call phys_to_dma in its implementation.
Introduce xen_dma_to_phys and call dma_to_phys in its implementation.
Call xen_phys_to_dma/xen_dma_to_phys instead of
xen_phys_to_bus/xen_bus_to_phys through swiotlb-xen.c.

Everything is taken care of by these changes except for
xen_swiotlb_alloc_coherent and xen_swiotlb_free_coherent, which need a
few explicit phys_to_dma/dma_to_phys calls.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Tested-by: Corey Minyard <cminyard@mvista.com>
Tested-by: Roman Shaposhnik <roman@zededa.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200710223427.6897-9-sstabellini@kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>