net: Introduce possible_net_t

Having to say
> #ifdef CONFIG_NET_NS
> 	struct net *net;
> #endif

in structures is a little bit wordy and a little bit error prone.

Instead it is possible to say:
> typedef struct {
> #ifdef CONFIG_NET_NS
>       struct net *net;
> #endif
> } possible_net_t;

And then in a header say:

> 	possible_net_t net;

Which is cleaner and easier to use and easier to test, as the
possible_net_t is always there no matter what the compile options.

Further this allows read_pnet and write_pnet to be functions in all
cases which is better at catching typos.

This change adds possible_net_t, updates the definitions of read_pnet
and write_pnet, updates optional struct net * variables that
write_pnet uses on to have the type possible_net_t, and finally fixes
up the b0rked users of read_pnet and write_pnet.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index cede40d..ddab1a2 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1721,9 +1721,7 @@
 	struct netpoll_info __rcu	*npinfo;
 #endif
 
-#ifdef CONFIG_NET_NS
-	struct net		*nd_net;
-#endif
+	possible_net_t			nd_net;
 
 	/* mid-layer private */
 	union {
@@ -1863,9 +1861,7 @@
 static inline
 void dev_net_set(struct net_device *dev, struct net *net)
 {
-#ifdef CONFIG_NET_NS
-	dev->nd_net = net;
-#endif
+	write_pnet(&dev->nd_net, net);
 }
 
 static inline bool netdev_uses_dsa(struct net_device *dev)