RDMA/ipoib: Use parent rdma device net namespace

Use the net namespace of the underlying rdma device.
After honoring the rdma device's namespace, the ipoib
netdev now also runs in the same net namespace of the
rdma device.

Add an API to read the net namespace of the rdma device
so that ULP such as IPoIB can use it to initialize its
netdev.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>