Fix: Don't fallback if subflow-only fin is acked

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: Correctly check for SYN-flag in mptcp_data_ack

The tcp_flags are not set for incoming packets. We thus cannot check
there for a SYN-packet. We have to check the tcp-header.

Similar fix in mptcp_skb_split_tail. The fin-flag is handled some lines
higher within this function.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: Initialize last_rbuf_opti correctly

It must be set to the correct timestamp as we create the mptcp_tcp_sock.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Update window on all subflows for fast recovery

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