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>
diff --git a/include/linux/if_link.h b/include/linux/if_link.h
index f4a2e6b..0ee969a 100644
--- a/include/linux/if_link.h
+++ b/include/linux/if_link.h
@@ -136,6 +136,7 @@
 	IFLA_PORT_SELF,
 	IFLA_AF_SPEC,
 	IFLA_GROUP,		/* Group the device belongs to */
+	IFLA_NET_NS_FD,
 	__IFLA_MAX
 };