xencomm: Fix a trivial warning.

Signed-off-by: Kouya Shimura <kouya@jp.fujitsu.com>

xen/common/xencomm.c

@@ -323,7 +323,7 @@ xencomm_copy_chunk_to(
                (unsigned long)xencomm_vaddr(paddr, page));
 
     memcpy(xencomm_vaddr(paddr, page), (void *)from, len);
-    xencomm_mark_dirty(xencomm_vaddr(paddr, page), len);
+    xencomm_mark_dirty((unsigned long)xencomm_vaddr(paddr, page), len);
     put_page(page);
 
     return 0;