sparc/iommu: fix __sbus_iommu_map_page for highmem pages

__sbus_iommu_map_page currently assumes all pages are mapped into the
kernel direct mapping.  Switch to using physical address instead of
virtual ones for all the normal mapping operations, and only use
the virtual addresses for cache flushing when not operating on
a highmem page.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>