mptcp: Correct support of TCP-Keepalive

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: Fix MPTCP FAST_CLOSE

We cannot let the Meta-socket be in close-state and continue keeping up
a timer. When the timer expires, the destruction will again call
tcp_done and mess everything up.

So, we now rather handle the retransmissions of the FAST_CLOSE in the
subflow's keepalive timer. That way we can effectively close the
meta-socket and just keep the subflow alive.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: tcp_current_mss cannot call mptcp_current_mss

mptcp_current_mss calls tcp_current_mss if no subflow is available with
the meta-sk as an argument. If then tcp_current_mss again redirects to
mptcp_current_mss, we are screwed and will loop forever...

Fixes: d1130dd78b (mptcp: Don't scheduler forward-retransmissions too often)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>