page-alloc: scrub pages used by hypervisor upon freeing

... unless they're part of a fully separate pool (and hence can't ever
be used for guest allocations).

This is XSA-100.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>

--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1409,7 +1409,10 @@ void free_xenheap_pages(void *v, unsigne
     pg = virt_to_page(v);
 
     for ( i = 0; i < (1u << order); i++ )
+    {
+        scrub_one_page(&pg[i]);
         pg[i].count_info &= ~PGC_xen_heap;
+    }
 
     free_heap_pages(pg, order);
 }
@@ -1579,6 +1582,8 @@ void free_domheap_pages(struct page_info
     else
     {
         /* Freeing anonymous domain-heap pages. */
+        for ( i = 0; i < (1 << order); i++ )
+            scrub_one_page(&pg[i]);
         free_heap_pages(pg, order);
         drop_dom_ref = 0;
     }