[SCTP] ICMP unreachable message reenables data transmit

[Schoch Christian]

During a measurement with CMT-SCTP and PF i figured out, that  
sometimes a ICMP Destination unreachable message triggers a message  
transmission on an inactive data path that has been primary before.

It looks as the ICMP message is reseting the inactive state back to  
active without reseting RTO.

This behavior is triggered by a returning heartbeat message when no  
ICMP unreachable by data is sent quite before.

Test system are two multi-homed hosts with FreeBSD8.1 and a WANem host  
between.

A wireshark log can be provided on demand (quite large).

Regards,
Schoch Christian