RDMA/core: Add a netlink command to change net namespace of rdma device

Provide an option to change the net namespace of a rdma device through a
netlink command. When multiple rdma devices exists in a system, and when
containers are used, this will limit rdma device visibility to a specified
net namespace.

An example command to change net namespace of mlx5_1 device to the
previously created net namespace 'foo' is:

$ ip netns add foo
$ rdma dev set mlx5_1 netns foo

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

include/uapi/rdma/rdma_netlink.h

@@ -469,12 +469,16 @@ enum rdma_nldev_attr {
 	 * either shared or exclusive among multiple net namespaces.
 	 */
 	RDMA_NLDEV_SYS_ATTR_NETNS_MODE,		/* u8 */
-
 	/*
 	 * Device protocol, e.g. ib, iw, usnic, roce and opa
 	 */
 	RDMA_NLDEV_ATTR_DEV_PROTOCOL,		/* string */
 
+	/*
+	 * File descriptor handle of the net namespace object
+	 */
+	RDMA_NLDEV_NET_NS_FD,			/* u32 */
+
 	/*
 	 * Always the end
 	 */