mptcp: Fix Bug #42: Do not reschedule tsq on closed socket

This socket anyway cannot send data anymore. Otherwise, if the sk_free
will indeed free the socket, we will bug in mptcp_sock_destruct as the
cb_list is not empty.

Reported-by: dykelin (https://github.com/dykelin)
Fixes: Bug #42 (https://github.com/multipath-tcp/mptcp/issues/42)
Fixes: be54ab36580a9 (Fix tsq-deferred)
Tested-by: dykelin (https://github.com/dykelin)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>