rdma: Add an option to set net namespace of rdma device

rdma: Add an option to set net namespace of rdma device

Enrich rdmatool with an option to set network namespace of RDMA
device. After successful execution of it, rdma device will
be accessible only in assigned network namespace.

rdma tool command examples and output.

First set netns mode to exclusive.

$ rdma system set netns exclusive

Now create network namespace and assign RDMA device to this
network namespace.

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

Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>