mptcp: Look for shortest distance in shortcut-list

We are looking for the shortcut, who has the shortest distance to our
skb, but is after our skb. Thus, shortcut->seq > our->seq. We measure
thus the distance as shortcut->seq - our->seq.

This was not done correctly previously.

Signed-off-by: Takumi Shinkai <shinkai@net.cne.okayama-u.ac.jp>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>