util/rb_tree: Reverse the order of comparison functions

The new order matches that of the comparison functions accepted by the C
standard library qsort() functions.  Being consistent with qsort will
hopefully help avoid developer confusion.

The only current user of the red-black tree is aub_mem.c which is pretty
easy to fix up.

Reviewed-by: Lionel Landwerlin <lionel.g.lndwerlin@intel.com>

src/util/rb_tree_test.c

@@ -56,7 +56,7 @@ static int
 rb_test_node_cmp_void(const struct rb_node *n, const void *v)
 {
     struct rb_test_node *tn = rb_node_data(struct rb_test_node, n, node);
-    return tn->key - *(int *)v;
+    return *(int *)v - tn->key;
 }
 
 static int
@@ -65,7 +65,7 @@ rb_test_node_cmp(const struct rb_node *a, const struct rb_node *b)
     struct rb_test_node *ta = rb_node_data(struct rb_test_node, a, node);
     struct rb_test_node *tb = rb_node_data(struct rb_test_node, b, node);
 
-    return ta->key - tb->key;
+    return tb->key - ta->key;
 }
 
 static void