net: Introduce read_pnet() and write_pnet() helpers

This patch introduces two helpers that deal with reading and writing
struct net pointers in various network structures.

Their implementation depends on CONFIG_NET_NS

For symmetry, both functions work with "struct net **pnet".

Their usage should reduce the number of #ifdef CONFIG_NET_NS,
without adding many helpers for each network structure
that hold a "struct net *pointer"
Signed-off-by: NEric Dumazet <dada1@cosmosbay.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -192,6 +192,24 @@ static inline void release_net(struct net *net)
 }
 #endif
 
+#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)
+
+#endif
 
 #define for_each_net(VAR)				\
 	list_for_each_entry(VAR, &net_namespace_list, list)