sctp: Make the proc files per network namespace.

- Convert all of the files under /proc/net/sctp to be per
  network namespace.

- Don't print anything for /proc/net/sctp/snmp except in
  the initial network namespaces as the snmp counters still
  have to be converted to be per network namespace.

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>