Internet: fragmented IPv6 PING replies are not received by the application

I tested (and confirmed it) for ICMP Echo, but I can not guarantee that UDP and TCP are not affected as well.

To check this bug it is enough to use examples/ipv6/fragmentation-ipv6.cc and enable

LogComponentEnable("Ping6Application", LOG_LEVEL_INFO);

Echo replies are not received by Ping6Application.