net: Tag the network flow with the network namespace it is in (v2)

As well as marking flows this indirectly marks the ipv4 routing cache as every routing entry contains a flow.

It is useful to add the network namespace into flows as frequently the routing information for ingoing and outgoing network packets is collected into a flow structure which is then used for several functions as it sorts out what is going on.

Changes from v1:
- remove flow.h dependency from net_namespace.h

---
 include/net/flow.h |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

index af59fa5..9590bbe 100644
@@ -10,7 +10,9 @@
 #include <linux/in6.h>
 #include <asm/atomic.h>
 
+struct net;
 struct flowi {
+	struct net *fl_net;
 	int	oif;
 	int	iif;
 	__u32	mark;

1.5.3.rc5