Build-Fix

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: Handle tcp_release_cb if subflow gets removed

Otherwise, mptcp_release_cb does not work, as the socket is no more part
of the list.

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

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: tcp_sendpage, if DSS-checksums are allowed.

We have to lock it, so that tcp_sendpage can unlock it again.

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