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>

include/net/net_namespace.h

@@ -231,24 +231,27 @@ int net_eq(const struct net *net1, const struct net *net2)
 #endif
 
 
+typedef struct {
 #ifdef CONFIG_NET_NS
-
-static inline void write_pnet(struct net **pnet, struct net *net)
-{
-	*pnet = net;
-}
-
-static inline struct net *read_pnet(struct net * const *pnet)
-{
-	return *pnet;
-}
-
-#else
-
-#define write_pnet(pnet, net)	do { (void)(net);} while (0)
-#define read_pnet(pnet)		(&init_net)
-
+	struct net *net;
 #endif
+} possible_net_t;
+
+static inline void write_pnet(possible_net_t *pnet, struct net *net)
+{
+#ifdef CONFIG_NET_NS
+	pnet->net = net;
+#endif
+}
+
+static inline struct net *read_pnet(const possible_net_t *pnet)
+{
+#ifdef CONFIG_NET_NS
+	return pnet->net;
+#else
+	return &init_net;
+#endif
+}
 
 #define for_each_net(VAR)				\
 	list_for_each_entry(VAR, &net_namespace_list, list)