net: sctp: find the correct highest_new_tsn in sack

Function sctp_check_transmitted(transport t, ...) would iterate all of
transport->transmitted queue and looking for the highest __newly__ acked tsn.
The original algorithm would depend on the order of the assoc->transport_list
(in function sctp_outq_sack line 1215 - 1226). The result might not be the
expected due to the order of the tranport_list.

Solution: checking if the exising is smaller than the new one before assigning

Signed-off-by: Chang Xiangzhong <changxiangzhong@gmail.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>