BUG/MINOR: tcp: make silent-drop always force a TCP reset

BUG/MINOR: tcp: make silent-drop always force a TCP reset

The silent-drop action is supposed to close with a TCP reset that is
either not sent or not too far. But since it's on the client-facing
side, the socket's lingering is enabled by default and the RST only
occurs if some pending unread data remain in the queue when closing.
This causes some clean shutdowns to occur with retransmits, which is
not good at all. Force linger_risk on the socket to flush all data
and destroy the socket.

No backport is needed, this was introduced in 1.6-dev6.