net: Make extern and export get_net_ns()

This function will be used to obtain net of tun device.

Signed-off-by: Kirill Tkhai <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/socket.h

···
 			  unsigned int flags, struct timespec *timeout);
 extern int __sys_sendmmsg(int fd, struct mmsghdr __user *mmsg,
 			  unsigned int vlen, unsigned int flags);
+
+extern struct ns_common *get_net_ns(struct ns_common *ns);
 #endif /* _LINUX_SOCKET_H */

net/socket.c

···
  *	what to do with it - that's up to the protocol still.
  */
 
-static struct ns_common *get_net_ns(struct ns_common *ns)
+struct ns_common *get_net_ns(struct ns_common *ns)
 {
 	return &get_net(container_of(ns, struct net, ns))->ns;
 }
+EXPORT_SYMBOL_GPL(get_net_ns);
 
 static long sock_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 {