intel-iommu: Fix boot inside 64bit virtualbox with io-apic disabled

Commit 074835f014 ("intel-iommu: Fix
kernel hand if interrupt remapping disabled in BIOS") is adding a check
for interrupt remapping disabled and is dereferencing the dmar_tbl
pointer without checking its value.

Unfortunately, this value is null when booting inside a 64bit virtual
box guest with io-apic disabled, leading to a crash. With a check on it,
the guest is now booting. It's triggering a WARN() in
clockevent_delta2ns but it's better than not booting at all and allows
the user to see there's something wrong on their io-apic setup.

Signed-off-by: Arnaud Patard <apatard@mandriva.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/pci/dmar.c

@@ -1466,5 +1466,7 @@ int __init dmar_ir_support(void)
 {
 	struct acpi_table_dmar *dmar;
 	dmar = (struct acpi_table_dmar *)dmar_tbl;
+	if (!dmar)
+		return 0;
 	return dmar->flags & 0x1;
 }