tcp: block connections in both directions

The packet can be retransmited after dumping the tcp connect.  The first
one is that the connection is blocked for only one direction. The second
one is that TCP timers continue work during dumping and they can send
packets. tcp_timestamp is saved for each tcp connections and then it’s
restored. So if a packet is sent after dumping, its timestamps is
saved by another side and this timestamp is sent back in the next packet
as the tsecr parameter. If this packet is received after restoring, it
looks like a packets from the future.

https://bugzilla.openvz.org/show_bug.cgi?id=2676



Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>