sctp: don't do RTT measurements with cookies

When receiving a cookie, the receiver does not know whether the
peer retransmitted the COOKIE-ECHO chunk or not. Therefore, don't
do an RTT measurement. It might be much too long.
To overcome this limitation, one could do at least two things:

1. Bundle the INIT-ACK chunk with a HEARTBEAT chunk for doing the RTT measurement. But this is not allowed.
2. Add a flag to the COOKIE-ECHO chunk, which indicates that it is the initial transmission, and not a retransmission. But this requires an RFC.

MFC after: 1 week