sctp: use the correct traffic class when sending SCTP/IPv6 packets

When sending packets the stcb was used to access the inp and then
access the endpoint specific IPv6 level options. This fails when
there exists an inp, but no stcb yet. This is the case for sending
an INIT-ACK in response to an INIT when no association already
exists. Fix this by just providing the inp instead of the stcb.

PR: 260120
MFC after: 1 week

(cherry picked from commit f32357be53d07622603049f9855be2d66ca6dc2b)