mptcp: rework DSS handling

Save DSS options in tcp_skb_cb when queing a packet to a subflow and
remove code that explictly copies them between skb copies.

Note that since we save the options in a union with skb_inet_parm we
need to clear them when we are done needing them (i.e. when we write
them to the TCP options) because the IP layer expects a clean
skb_inet_parm.

Also it is safe to union them with path_mask since path_mask is not
used after queing the packet to the subflow.

Depends-on: 582e3bdae0117 (mptcp: create helper functions to deal with DSS options)
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>