Fix: Clone reinjected skb's in mptcp_skb_entail

We have to clone them, because after the call to tcp_transmit_skb, we
need easy-access to the data-sequence number.
Cloning the skb is only a minor performance penalty, as reinjections are
rather seldom, and it facilitates the subsequent code.

This patch also removes some very obsolete comments.

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

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>