powerpc/iommu: Prevent false TCE leak message

When a device DMA window includes the address 0, it's reserved in the
TCE bitmap to avoid returning that address to drivers.

When the device is removed, the bitmap is checked for any mappings not
removed by the driver, indicating a possible DMA mapping leak. Since the
reserved address is not cleared, a message is printed, warning of such a
leak.

Check for the reservation, and clear it before checking for any other
standing mappings.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>