sctp: implement socket option SCTP_GET_ASSOC_ID_LIST

This patch Implement socket option SCTP_GET_ASSOC_ID_LIST.
SCTP Socket API Extension:

  8.2.6. Get the Current Identifiers of Associations
         (SCTP_GET_ASSOC_ID_LIST)

  This option gets the current list of SCTP association identifiers of
  the SCTP associations handled by a one-to-many style socket.

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