SCTP: Allow ADD_IP to work with AUTH for backward compatibility.

This patch adds a tunable that will allow ADD_IP to work without
AUTH for backward compatibility.  The default value is off since
the default value for ADD_IP is off as well.  People who need
to use ADD-IP with older implementations take risks of connection
hijacking and should consider upgrading or turning this tunable on.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>