sctp: fix compile issue with disabled CONFIG_NET_NS

struct seq_net_private has no struct net
if CONFIG_NET_NS is not enabled

Signed-off-by: Ulrich Weber <ulrich.weber@sophos.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>