net: move and export get_net_ns_by_pid

The function get_net_ns_by_pid(), to get a network
namespace from a pid_t, will be required in cfg80211
as well. Therefore, let's move it to net_namespace.c
and export it. We can't make it a static inline in
the !NETNS case because it needs to verify that the
given pid even exists (and return -ESRCH).

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -108,6 +108,8 @@ 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);
+
 #ifdef CONFIG_NET_NS
 extern void __put_net(struct net *net);