sparc/iommu: use !PageHighMem to check if a page has a kernel mapping

This deobsfucates the check a bit, and prepares for future changes.

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>