net: Allow setting the network namespace by fd

Take advantage of the new abstraction and allow network devices
to be placed in any network namespace that we have a fd to talk
about.

Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Daniel Lezcano <daniel.lezcano@free.fr>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

include/net/net_namespace.h

@@ -119,6 +119,7 @@ static inline struct net *copy_net_ns(unsigned long flags, struct net *net_ns)
 extern struct list_head net_namespace_list;
 
 extern struct net *get_net_ns_by_pid(pid_t pid);
+extern struct net *get_net_ns_by_fd(int pid);
 
 #ifdef CONFIG_NET_NS
 extern void __put_net(struct net *net);