[mle] update router table after adding a new neighbor (#10288)

Use the `Route64 TLV` to update the router table after adding a new
neighbor in `HandleLinkAccept`.

With this change, when a router re-joins the network, it will restore
its nexthop table while processing the `link accept`, rather than
having to wait for an `mle advertisement`.

src/core/thread/mle_router.cpp

@@ -886,6 +886,7 @@ Error MleRouter::HandleLinkAccept(RxInfo &aRxInfo, bool aRequest)
     RouteTlv        routeTlv;
     LeaderData      leaderData;
     uint8_t         linkMargin;
+    bool            shouldUpdateRoutes = false;
 
     SuccessOrExit(error = Tlv::Find<SourceAddressTlv>(aRxInfo.mMessage, sourceAddress));
 
@@ -972,6 +973,7 @@ Error MleRouter::HandleLinkAccept(RxInfo &aRxInfo, bool aRequest)
         mLinkRequestAttempts    = 0;
         mRetrieveNewNetworkData = true;
         IgnoreError(SendDataRequest(aRxInfo.mMessageInfo.GetPeerAddr()));
+        shouldUpdateRoutes = true;
 
 #if OPENTHREAD_CONFIG_TIME_SYNC_ENABLE
         Get<TimeSync>().HandleTimeSyncMessage(aRxInfo.mMessage);
@@ -1007,8 +1009,7 @@ Error MleRouter::HandleLinkAccept(RxInfo &aRxInfo, bool aRequest)
                 router = mRouterTable.FindRouterById(routerId);
                 OT_ASSERT(router != nullptr);
             }
-
-            mRouterTable.UpdateRoutes(routeTlv, routerId);
+            shouldUpdateRoutes = true;
             break;
 
         case kErrorNotFound:
@@ -1043,6 +1044,11 @@ Error MleRouter::HandleLinkAccept(RxInfo &aRxInfo, bool aRequest)
 
     mNeighborTable.Signal(NeighborTable::kRouterAdded, *router);
 
+    if (shouldUpdateRoutes)
+    {
+        mRouterTable.UpdateRoutes(routeTlv, routerId);
+    }
+
     aRxInfo.mClass = RxInfo::kAuthoritativeMessage;
     ProcessKeySequence(aRxInfo);