Send an ICMPv6 PacketTooBig message in case of forwading a packet which
is too big for the outgoing interface and no firewall is involed.
This problem was introduced in
https://svnweb.freebsd.org/changeset/base/324996
Thanks to Irene Ruengeler for finding the bug and testing the fix.

Reviewed by: kp@
MFC after: 3 days