[mle] kAppendMeshLocalOnly for Child Update Request when detached (#5295)

When the child reboot, only register the mesh local address to avoid
possible large Child Update Response which would be fragmented, thus
avoiding reattach process.

src/core/thread/mle.cpp

@@ -2220,9 +2220,10 @@ exit:
 
 otError Mle::SendChildUpdateRequest(void)
 {
-    otError      error = OT_ERROR_NONE;
-    Ip6::Address destination;
-    Message *    message = nullptr;
+    otError                 error = OT_ERROR_NONE;
+    Ip6::Address            destination;
+    Message *               message = nullptr;
+    AddressRegistrationMode mode    = kAppendAllAddresses;
 
     if (!mParent.IsStateValidOrRestoring())
     {
@@ -2239,16 +2240,12 @@ otError Mle::SendChildUpdateRequest(void)
     SuccessOrExit(error = AppendHeader(*message, Header::kCommandChildUpdateRequest));
     SuccessOrExit(error = AppendMode(*message, mDeviceMode));
 
-    if (!IsFullThreadDevice())
-    {
-        SuccessOrExit(error = AppendAddressRegistration(*message));
-    }
-
     switch (mRole)
     {
     case kRoleDetached:
         mParentRequestChallenge.GenerateRandom();
         SuccessOrExit(error = AppendChallenge(*message, mParentRequestChallenge));
+        mode = kAppendMeshLocalOnly;
         break;
 
     case kRoleChild:
@@ -2264,6 +2261,11 @@ otError Mle::SendChildUpdateRequest(void)
         OT_UNREACHABLE_CODE(break);
     }
 
+    if (!IsFullThreadDevice())
+    {
+        SuccessOrExit(error = AppendAddressRegistration(*message, mode));
+    }
+
     destination.SetToLinkLocalAddress(mParent.GetExtAddress());
     SuccessOrExit(error = SendMessage(*message, destination));