dccp: Add check for sequence number in ICMPv6 message

This adds a sequence number check for ICMPv6 DCCP error packets, in the same
manner as it has been done for ICMPv4 in the previous patch.

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