sctp: Fix SCTP_MAXSEG socket option to comply to spec.
We had a bug that we never stored the user-defined value for MAXSEG when setting the value on an association. Thus future PMTU events ended up re-writing the frag point and increasing it past user limit. Additionally, when setting the option on the socket/endpoint, we effect all current associations, which is against spec. Now, we store the user 'maxseg' value along with the computed 'frag_point'. We inherit 'maxseg' from the socket at association creation and use it as an upper limit for 'frag_point' when its set. Signed-off-by: Vlad Yasevich <firstname.lastname@example.org>