When processing an ICMP packet containing an SCTP packet, it
is required to check the verification tag. However, this
requires the verification tag to be not 0. Enforce this.
For packets with a verification tag of 0, we need to
check it it contains an INIT chunk and use the initiate
tag for the validation. This will be a separate commit,
since it touches also other code.

MFC after: 1 week