swiotlb: store phys address in io_tlb_orig_addr array

Impact: refactor code, cleanup

When we enable swiotlb for platforms that support HIGHMEM, we
can no longer store the virtual address of the original dma
buffer, because that buffer might not have a permament mapping.

Change the swiotlb code to instead store the physical address of
the original buffer.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>