Fix: Don't account subflow-fin when checking for valid mapping

The subflow-fin is not part of the DSS-mapping. Thus, when calculating
the end_seq of the TCP-space during mapping-checks, we have to take this
into account.

Reported-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>