Plane: disallow mavlink disarm while flying

this relies on is_flying(), and we will need to watch for reports of
the heuristics failing

ArduPlane/AP_Arming.cpp

@@ -275,12 +275,15 @@ bool AP_Arming_Plane::arm(const AP_Arming::Method method, const bool do_arming_c
 bool AP_Arming_Plane::disarm(const AP_Arming::Method method, bool do_disarm_checks)
 {
     if (do_disarm_checks &&
-        method == AP_Arming::Method::RUDDER) {
-        // don't allow rudder-disarming in flight:
+        (method == AP_Arming::Method::MAVLINK ||
+         method == AP_Arming::Method::RUDDER)) {
         if (plane.is_flying()) {
-            // obviously this could happen in-flight so we can't warn about it
+            // don't allow mavlink or rudder disarm while flying
             return false;
         }
+    }
+    
+    if (do_disarm_checks && method == AP_Arming::Method::RUDDER) {
         // option must be enabled:
         if (get_rudder_arming_type() != AP_Arming::RudderArming::ARMDISARM) {
             gcs().send_text(MAV_SEVERITY_INFO, "Rudder disarm: disabled");