[POWERPC] pseries: Handle null iommu dma-window property correctly

Some versions of pSeries firmware fail to set up a
dma-window property for PCI slots that are unoccupied.
As a result, the loop searching for this propery, in
pci_dma_dev_setup_pSeriesLP(), can run to the end, resulting
in a NULL pointer dereference later in the routine. This
patch prevents the crash, and prints a warning message.

This is theoretically a rare error, as it occurs on what
is hopefully just beta levels of firmware. But just in case
this firmware escapes into the wild, this patch will avoid
the crash.
Signed-off-by: NLinas Vepstas <linas@austin.ibm.com>

arch/powerpc/platforms/pseries/iommu.c

@@ -504,6 +504,12 @@ static void pci_dma_dev_setup_pSeriesLP(struct pci_dev *dev)
 			break;
 	}
 
+	if (!pdn || !PCI_DN(pdn)) {
+		printk(KERN_WARNING "pci_dma_dev_setup_pSeriesLP: "
+		       "no DMA window found for pci dev=%s dn=%s\n",
+				 pci_name(dev), dn? dn->full_name : "<null>");
+		return;
+	}
 	DBG("  parent is %s\n", pdn->full_name);
 
 	/* Check for parent == NULL so we don't try to setup the empty EADS