mptcp: Change sk_shutdown when combining dfin with sub-fin

Because, this is just like a call to shutdown(). So, we have to set
sk_shutdown correctly.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: Force close a subflow when resetting fallbacked MPTCP session

We have to force the closure of the subflow, because the connection is
completly dead now.

Reported-by: Ardhan Madras <ardhan@rocksis.net>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>