PCI: Fail pci_map_rom() if the option ROM is invalid

If we detect a invalid PCI option ROM (e.g., invalid ROM header signature),
we should unmap it immediately and fail. It doesn't make any sense to
return a mapped area with size of 0.

I have seen this case on Intel GVTg vGPU, which has no VBIOS. It will not
cause a real problem, but we should skip it as early as possible.
Signed-off-by: NChangbin Du <changbin.du@intel.com>
[bhelgaas: split non-functional change into separate patch]
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

drivers/pci/rom.c

@@ -156,8 +156,13 @@ void __iomem *pci_map_rom(struct pci_dev *pdev, size_t *size)
 	 * True size is important if the ROM is going to be copied.
 	 */
 	*size = pci_get_rom_size(pdev, rom, *size);
+	if (!*size)
+		goto invalid_rom;
+
 	return rom;
 
+invalid_rom:
+	iounmap(rom);
 err_ioremap:
 	/* restore enable if ioremap fails */
 	if (!(res->flags & IORESOURCE_ROM_ENABLE))