sctp: Fix ECN markings for IPv6

Commit e9df2e8fd8fbc95c57dbd1d33dada66c4627b44c ("[IPV6]: Use
appropriate sock tclass setting for routing lookup.") also changed the
way that ECN capable transports mark this capability in IPv6.  As a
result, SCTP was not marking ECN capablity because the traffic class
was never set.  This patch brings back the markings for IPv6 traffic.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>