powerpc/eeh: Fix possible null deref in eeh_dump_dev_log()

If an error occurs during an unplug operation, it's possible for
eeh_dump_dev_log() to be called when edev->pdn is null, which
currently leads to dereferencing a null pointer.

Handle this by skipping the error log for those devices.
Signed-off-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/eeh.c

@@ -169,6 +169,11 @@ static size_t eeh_dump_dev_log(struct eeh_dev *edev, char *buf, size_t len)
 	int n = 0, l = 0;
 	char buffer[128];
 
+	if (!pdn) {
+		pr_warn("EEH: Note: No error log for absent device.\n");
+		return 0;
+	}
+
 	n += scnprintf(buf+n, len-n, "%04x:%02x:%02x.%01x\n",
 		       pdn->phb->global_number, pdn->busno,
 		       PCI_SLOT(pdn->devfn), PCI_FUNC(pdn->devfn));