sctp: Enable sctp in all network namespaces

- Fix the sctp_af operations to work in all namespaces
- Enable sctp socket creation in all network namespaces.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>