dccp: Fix sequence number check for ICMPv4 packets

The payload of ICMP message is a part of the packet sent by ourself,
so the sequence number check must use AWL and AWH, not SWL and SWH.

For example:
     Endpoint A                  Endpoint B

     DATA-ACK       -------->
     (SEQ=X)
                    <--------    ICMP (Fragmentation Needed)
                                 (SEQ=X)

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>