[NETNS][DST]: Add the network namespace pointer in dst_ops

The network namespace pointer can be stored into the dst_ops structure.
This is usefull when there are multiple instances of the dst_ops for a
protocol. When there are no several instances, this field will be never
used in the protocol. So there is no impact for the protocols which do
implement the network namespaces.
Signed-off-by: NDaniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -102,6 +102,7 @@ struct dst_ops
 
 	atomic_t		entries;
 	struct kmem_cache 		*kmem_cachep;
+	struct net              *dst_net;
 };
 
 #ifdef __KERNEL__