SCTP: drop SACK if ctsn is not less than the next tsn of assoc

We need to drop the SACK if the peer is attempting to acknowledge
unset data, i.e.  the CTSN in the SACK is greater or equal to the
next TSN we will send.

Example:
Endpoint A                                      Endpoint B
                             <---------------   DATA (TSN=1)
SACK(TSN=1) ---------------> 
                             <---------------   DATA (TSN=2)
                             <---------------   DATA (TSN=3)
                             <---------------   DATA (TSN=4)
                             <---------------   DATA (TSN=5)
SACK(TSN=1000) --------------->
                             <---------------   DATA (TSN=6)
                             <---------------   DATA (TSN=7)

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>