Fixed Icmpv6L4Protocol messages being sent with HopLimit of DefaultTtl from Ipv6L3Protocol

[Afonso Oliveira]

Dear ns-developers,

I have submited a minor fix for review of a problem I found during tests with external nodes to ns-3. Icmpv6L4Protocol::SendMessage was using Ipv4 SocketIpTtl Tag as opposed to the SocketIpv6HopLimitTag. When these ICMPv6 packets were going through the Ipv6L3Protocol class, the SocketIpv6HopLimitTag was not found, resulting in being sent with the DefaultTtl value of 64, which must not be done according to the RFC.

Issue: https://codereview.appspot.com/353770043

Regards,

Afonso Oliveira