mptcp: Fix Bug#24: Correctly handle closed-sockets in tcp_rcv_state_process

While merging to 3.11, we messed up one case-statement of the switch in
tcp_rcv_state_process. This patch moves the case-statement again where
it belongs.

Reported-by: David Heidelberger <david.heidelberger@ixit.cz>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>