PCI/MSI: Check that we have a legacy interrupt line before using it

It seems like there are some devices (e.g. the PCIe root port driver) that
may not always have a INTx interrupt.  Check for dev->irq before returning
a legacy interrupt in pci_irq_alloc_vectors to properly handle this case.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

drivers/pci/msi.c

@@ -1198,9 +1198,11 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
 	}
 
 	/* use legacy irq if allowed */
-	if ((flags & PCI_IRQ_LEGACY) && min_vecs == 1) {
-		pci_intx(dev, 1);
-		return 1;
+	if (flags & PCI_IRQ_LEGACY) {
+		if (min_vecs == 1 && dev->irq) {
+			pci_intx(dev, 1);
+			return 1;
+		}
 	}
 
 	return vecs;