mptcp: Allow receiving splitted data-fin without mapping on last skb

It may be, that we receive a mapping covering two segments. This mapping
may include a DATA_FIN and it may be that the last of these two segments
does not have a DSS-option. Then, mptcp_is_data_fin2() will always be
false and we do not set the fin-bit on the segment.

Fixes: 47080a0c11823 (Simplify tcp read code from application)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>