[NETFILTER]: conntrack: add fixed timeout flag in connection tracking

Add a flag in a connection status to have a non updated timeout.
This permits to have connection that automatically die at a given
time.

Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>