swiotlb: Make io_tlb_start a physical address instead of a virtual one

swiotlb: Make io_tlb_start a physical address instead of a virtual one

This change replaces all references to the virtual address for io_tlb_start
with references to the physical address io_tlb_end.  The main advantage of
replacing the virtual address with a physical address is that we can avoid
having to do multiple translations from the virtual address to the physical
one needed for testing an existing DMA address.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>