x86/mm: Fix infinite loop in get_spage_pages()

c/s 2b8eb37 switched int i to being unsigned, but the undo logic on failure
relied in i being signed.  As i being unsigned in still preforable, adjust the
undo logic to work with an unsigned i.

Coverity-ID: 1413017
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Konrad Rzeszutek Will <konrad.wilk@oracle.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

xen/arch/x86/mm.c

@@ -2687,7 +2687,7 @@ static int get_spage_pages(struct page_info *page, struct domain *d)
     {
         if ( !get_page_and_type(page, d, PGT_writable_page) )
         {
-            while ( --i >= 0 )
+            while ( i-- > 0 )
                 put_page_and_type(--page);
             return 0;
         }