RFC4340 said:
  8.5.  Pseudocode
       ...
       If P.type is not Data, Ack, or DataAck and P.X == 0 (the packet
             has short sequence numbers), drop packet and return

But DCCP has some mistake to handle short sequence numbers packet, now
it drop packet only if P.type is Data, Ack, or DataAck and P.X == 0.
Signed-off-by: NWei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>