sctp: add Adaptation Layer Indication parameter only when it's set

RFC5061 states:

        Each adaptation layer that is defined that wishes
        to use this parameter MUST specify an adaptation code point in an
        appropriate RFC defining its use and meaning.

If the user has not set one - assume they don't want to sent the param
with a zero Adaptation Code Point.

Rationale - Currently the IANA defines zero as reserved - and
1 as the only valid value - so we consider zero to be unset - to save
adding a boolean to the socket structure.

Including this parameter unconditionally causes endpoints that do not
understand it to report errors unnecessarily.

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