ipv4: Namespecify TCP PMTU mechanism

Packetization Layer Path MTU Discovery works separately beside
Path MTU Discovery at IP level, different net namespace has
various requirements on which one to chose, e.g., a virutalized
container instance would require TCP PMTU to probe an usable
effective mtu for underlying tunnel, while the host would
employ classical ICMP based PMTU to function.

Hence making TCP PMTU mechanism per net namespace to decouple
two functionality. Furthermore the probe base MSS should also
be configured separately for each namespace.

Signed-off-by: Fan Du <fan.du@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/ipv4.h

@@ -82,6 +82,8 @@ struct netns_ipv4 {
 
 	int sysctl_fwmark_reflect;
 	int sysctl_tcp_fwmark_accept;
+	int sysctl_tcp_mtu_probing;
+	int sysctl_tcp_base_mss;
 
 	struct ping_group_range ping_group_range;