vt-d: Fixpanic in msi_msg_read_remap_rte with acpi=off

Xen panics when "acpi=off noacpi" is set. Problem is caused by
dereferencing NULL pointer in drhd after calling
acpi_find_matched_drhd_unit. As acpi_find_matched_drhd_unit can
return NULL, checks has to be done before returned value is used.

From: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Keir Fraser <keir.fraser@eu.citrix.com>

xen/drivers/passthrough/vtd/intremap.c

@@ -563,7 +563,8 @@ void msi_msg_read_remap_rte(
     struct iommu *iommu = NULL;
     struct ir_ctrl *ir_ctrl;
 
-    drhd = acpi_find_matched_drhd_unit(pdev);
+    if ( (drhd = acpi_find_matched_drhd_unit(pdev)) == NULL )
+        return;
     iommu = drhd->iommu;
 
     ir_ctrl = iommu_ir_ctrl(iommu);
@@ -581,7 +582,8 @@ void msi_msg_write_remap_rte(
     struct iommu *iommu = NULL;
     struct ir_ctrl *ir_ctrl;
 
-    drhd = acpi_find_matched_drhd_unit(pdev);
+    if ( (drhd = acpi_find_matched_drhd_unit(pdev)) == NULL )
+        return;
     iommu = drhd->iommu;
 
     ir_ctrl = iommu_ir_ctrl(iommu);

xen/drivers/passthrough/vtd/iommu.c

@@ -1343,7 +1343,8 @@ static int reassign_device_ownership(
     if (!pdev)
         return -ENODEV;
 
-    drhd = acpi_find_matched_drhd_unit(pdev);
+    if ( (drhd = acpi_find_matched_drhd_unit(pdev)) == NULL )
+        return -ENODEV;
     pdev_iommu = drhd->iommu;
     domain_context_unmap(source, bus, devfn);
 
@@ -1357,7 +1358,7 @@ static int reassign_device_ownership(
     for_each_pdev ( source, pdev )
     {
         drhd = acpi_find_matched_drhd_unit(pdev);
-        if ( drhd->iommu == pdev_iommu )
+        if ( drhd && drhd->iommu == pdev_iommu )
         {
             found = 1;
             break;