ipv4: Namespaceify tcp_keepalive_time sysctl knob

Different net namespaces might have different requirements as to
the keepalive time of tcp sockets. This might be required in cases
where different firewall rules are in place which require tcp
timeout sockets to be increased/decreased independently of the host.

Signed-off-by: Nikolay Borisov <kernel@kyup.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/ipv4.h

@@ -94,6 +94,8 @@ struct netns_ipv4 {
 	int sysctl_tcp_probe_threshold;
 	u32 sysctl_tcp_probe_interval;
 
+	int sysctl_tcp_keepalive_time;
+
 	struct ping_group_range ping_group_range;
 
 	atomic_t dev_addr_genid;