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 <vladislav.yasevich@hp.com>

include/net/sctp/structs.h

@@ -1763,6 +1763,7 @@ struct sctp_association {
 
 	/* The message size at which SCTP fragmentation will occur. */
 	__u32 frag_point;
+	__u32 user_frag;
 
 	/* Counter used to count INIT errors. */
 	int init_err_counter;