honor MEMF_no_refcount in alloc_heap_pages()

Non-anonymous allocations with this flag set should - for the purpose
of the availability check - be treated just like anonymous ones, as
they wouldn't lead to a reduction of ->outstanding_pages.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -617,7 +617,8 @@ static struct page_info *alloc_heap_page
      */
     if ( (outstanding_claims + request >
           total_avail_pages + tmem_freeable_pages()) &&
-          (d == NULL || d->outstanding_pages < request) )
+          ((memflags & MEMF_no_refcount) ||
+           !d || d->outstanding_pages < request) )
         goto not_found;
 
     /*