sparc/iommu: move per-page flushing into __sbus_iommu_map_page

This prepares for reusing __sbus_iommu_map_page in the map_sg path.

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>