exec.c: get nodes_nb_alloc with one MAX calculation

The purpose of these two MAX here is to get the maximum of these three
variables:

    A: map->nodes_nb + nodes
    B: map->nodes_nb_alloc
    C: alloc_hint

We can write it like MAX(A, B, C). Since the if condition says A > B,
this means MAX(A, B, C) = MAX(A, C).

This patch just simplify the calculation a bit.
Signed-off-by: NWei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190321082555.21118-4-richardw.yang@linux.intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -227,8 +227,7 @@ static void phys_map_node_reserve(PhysPageMap *map, unsigned nodes)
 {
     static unsigned alloc_hint = 16;
     if (map->nodes_nb + nodes > map->nodes_nb_alloc) {
-        map->nodes_nb_alloc = MAX(map->nodes_nb_alloc, alloc_hint);
-        map->nodes_nb_alloc = MAX(map->nodes_nb_alloc, map->nodes_nb + nodes);
+        map->nodes_nb_alloc = MAX(alloc_hint, map->nodes_nb + nodes);
         map->nodes = g_renew(Node, map->nodes, map->nodes_nb_alloc);
         alloc_hint = map->nodes_nb_alloc;
     }