iommu/amd-vi: avoid pointless flushes in invalidate_all_domain_pages()

Fix invalidate_all_domain_pages() to only attempt to flush the domains
that have IOMMU enabled, otherwise the flush is pointless.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/drivers/passthrough/amd/iommu_init.c

@@ -1532,8 +1532,10 @@ int __init amd_iommu_init_late(void)
 static void invalidate_all_domain_pages(void)
 {
     struct domain *d;
+
     for_each_domain( d )
-        amd_iommu_flush_all_pages(d);
+        if ( is_iommu_enabled(d) )
+            amd_iommu_flush_all_pages(d);
 }
 
 static int cf_check _invalidate_all_devices(