netns: embed ip6_dst_ops directly

struct net::ipv6.ip6_dst_ops is separatedly dynamically allocated,
but there is no fundamental reason for it. Embed it directly into
struct netns_ipv6.

For that:
* move struct dst_ops into separate header to fix circular dependencies
	I honestly tried not to, it's pretty impossible to do other way
* drop dynamical allocation, allocate together with netns

For a change, remove struct dst_ops::dst_net, it's deducible
by using container_of() given dst_ops pointer.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/dst.h b/include/net/dst.h
index 7fc409c..5a900dd 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -8,6 +8,7 @@
 #ifndef _NET_DST_H
 #define _NET_DST_H
 
+#include <net/dst_ops.h>
 #include <linux/netdevice.h>
 #include <linux/rtnetlink.h>
 #include <linux/rcupdate.h>
@@ -102,28 +103,6 @@
 	};
 };
 
-
-struct dst_ops
-{
-	unsigned short		family;
-	__be16			protocol;
-	unsigned		gc_thresh;
-
-	int			(*gc)(struct dst_ops *ops);
-	struct dst_entry *	(*check)(struct dst_entry *, __u32 cookie);
-	void			(*destroy)(struct dst_entry *);
-	void			(*ifdown)(struct dst_entry *,
-					  struct net_device *dev, int how);
-	struct dst_entry *	(*negative_advice)(struct dst_entry *);
-	void			(*link_failure)(struct sk_buff *);
-	void			(*update_pmtu)(struct dst_entry *dst, u32 mtu);
-	int			(*local_out)(struct sk_buff *skb);
-
-	atomic_t		entries;
-	struct kmem_cache 		*kmem_cachep;
-	struct net              *dst_net;
-};
-
 #ifdef __KERNEL__
 
 static inline u32