xen: re-enable boot-time ballooning

Now that the balloon driver doesn't stumble over non-RAM pages, we
can enable the extra space for ballooning.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/setup.c

@@ -248,8 +248,7 @@ char * __init xen_memory_setup(void)
 	else
 		extra_pages = 0;
 
-	if (!xen_initial_domain())
-		xen_add_extra_mem(extra_pages);
+	xen_add_extra_mem(extra_pages);
 
 	return "Xen";
 }