iommu/vt-d: Check for NULL pointer in dmar_acpi_dev_scope_init()

When ir_dev_scope_init() is called via a rootfs initcall it
will check for irq_remapping_enabled before it calls
(indirectly) into dmar_acpi_dev_scope_init() which uses the
dmar_tbl pointer without any checks.

The AMD IOMMU driver also sets the irq_remapping_enabled
flag which causes the dmar_acpi_dev_scope_init() function to
be called on systems with AMD IOMMU hardware too, causing a
boot-time kernel crash.
Signed-off-by: Joerg Roedel <joro@8bytes.org>

drivers/iommu/dmar.c

@@ -657,7 +657,12 @@ static void __init dmar_acpi_insert_dev_scope(u8 device_number,
 
 static int __init dmar_acpi_dev_scope_init(void)
 {
-	struct acpi_dmar_andd *andd = (void *)dmar_tbl + sizeof(struct acpi_table_dmar);
+	struct acpi_dmar_andd *andd;
+
+	if (dmar_tbl == NULL)
+		return -ENODEV;
+
+	andd = (void *)dmar_tbl + sizeof(struct acpi_table_dmar);
 
 	while (((unsigned long)andd) <
 	       ((unsigned long)dmar_tbl) + dmar_tbl->length) {