xen/page_alloc: scrub each page in unprepare_staticmem_pages

unprepare_staticmem_pages() can unprepare multiple pages in a static
memory range. When need_scrub is true, each page must be scrubbed
before it is marked static.

The loop currently passes the base pointer pg to scrub_one_page() on
every iteration. For a multi-page range, this repeatedly scrubs page 0
and leaves the remaining pages unscrubbed, so a guest can later read
stale data from pages 1..nr_mfns-1 of a static or static-shared bank.

Pass &pg[i] to scrub_one_page() so each iteration scrubs the page it
is processing. This ensures every page in the range is scrubbed before
it is marked static.

Fixes: 4a9e73e6e5 ("xen/arm: static memory initialization")
Signed-off-by: Peter Kang <peter.kang@boeing.com>

xen/common/page_alloc.c

@@ -2988,7 +2988,7 @@ void unprepare_staticmem_pages(struct page_info *pg, unsigned long nr_mfns,
         if ( need_scrub )
         {
             /* TODO: asynchronous scrubbing for pages of static memory. */
-            scrub_one_page(pg, true);
+            scrub_one_page(&pg[i], true);
         }
 
         pg[i].count_info |= PGC_static;