memory: fix XSA-158 fix

For one the uses of domu_max_order and ptdom_max_order were swapped.

And then gcc warns about an unused result of a __must_check function
in the control part of a conditional expression when both other
expressions can be determined by the compiler to produce the same value
(see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68039), which happens
when HAS_PASSTHROUGH is undefined (i.e. for ARM on 4.4 and older).

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

--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -55,8 +55,6 @@ static unsigned int __read_mostly ctldom
 static unsigned int __read_mostly hwdom_max_order = CONFIG_HWDOM_MAX_ORDER;
 #ifdef HAS_PASSTHROUGH
 static unsigned int __read_mostly ptdom_max_order = CONFIG_PTDOM_MAX_ORDER;
-#else
-# define ptdom_max_order domu_max_order
 #endif
 static void __init parse_max_order(const char *s)
 {
@@ -75,8 +73,12 @@ custom_param("memop-max-order", parse_ma
 
 static unsigned int max_order(const struct domain *d)
 {
-    unsigned int order = cache_flush_permitted(d) ? domu_max_order
-                                                  : ptdom_max_order;
+    unsigned int order = domu_max_order;
+
+#ifdef HAS_PASSTHROUGH
+    if ( cache_flush_permitted(d) && order < ptdom_max_order )
+        order = ptdom_max_order;
+#endif
 
     if ( is_control_domain(d) && order < ctldom_max_order )
         order = ctldom_max_order;