mptcp: Fix shift-sack-data for MPTCP

If we are shifting sack'd data, we are removing one skb from the
write-queue. This skb will be holding a DSS-option in tcp_skb_cb->dss.

By removing the skb, the dss-option is lost forever and (may) never
reach the receiver.

So, we cannot shift sack'd data.

Fixes: f17b29179f97e (TSO Support for MPTCP)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>