[NET]: netlink support for moving devices between network namespaces.

The simplest thing to implement is moving network devices between
namespaces.  However with the same attribute IFLA_NET_NS_PID we can
easily implement creating devices in the destination network
namespace as well.  However that is a little bit trickier so this
patch sticks to what is simple and easy.

A pid is used to identify a process that happens to be a member
of the network namespace we want to move the network device to.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_link.h

@@ -78,6 +78,7 @@ enum
 	IFLA_LINKMODE,
 	IFLA_LINKINFO,
 #define IFLA_LINKINFO IFLA_LINKINFO
+	IFLA_NET_NS_PID,
 	__IFLA_MAX
 };