sctp: Support the new specification of sctp_connectx()

The specification of sctp_connectx() has been changed to return
an association id.  We've added a new socket option that will
return the association id as the return value from the setsockopt()
call.  The library that implements sctp_connectx() interface will
implement both socket options.

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