Copter: Refactor fence.get_safe_alt() to fence.get_safe_alt_max()

ArduCopter/control_rtl.cpp

@@ -482,7 +482,7 @@ void Copter::rtl_compute_return_target(bool terrain_following_allowed)
     if ((fence.get_enabled_fences() & AC_FENCE_TYPE_ALT_MAX) != 0) {
         // get return target as alt-above-home so it can be compared to fence's alt
         if (rtl_path.return_target.get_alt_cm(Location_Class::ALT_FRAME_ABOVE_HOME, target_alt)) {
-            float fence_alt = fence.get_safe_alt()*100.0f;
+            float fence_alt = fence.get_safe_alt_max()*100.0f;
             if (target_alt > fence_alt) {
                 // reduce target alt to the fence alt
                 rtl_path.return_target.alt -= (target_alt - fence_alt);