DataFlash: eliminate calls to manipulate _writes_enabled

libraries/DataFlash/DataFlash.h

@@ -212,6 +212,7 @@ public:
     bool logging_failed() const;
 
     void set_vehicle_armed(bool armed_state);
+    bool vehicle_is_armed() const { return _armed; }
 
 protected:
 

libraries/DataFlash/DataFlash_Backend.cpp

@@ -265,5 +265,9 @@ bool DataFlash_Backend::WritesOK() const
     if (!_writes_enabled) {
         return false;
     }
+    if (!_front.vehicle_is_armed() && !_front.log_while_disarmed()) {
+        return false;
+    }
+
     return true;
 }