RDMA/core: Support netlink commands in non init_net net namespaces

Now that IB core supports RDMA device binding with specific net namespace,
enable IB core to accept netlink commands in non init_net namespaces.

This is done by having per net namespace netlink socket.

At present only netlink device handling client RDMA_NL_NLDEV supports
device handling in multiple net namespaces.  Hence do not accept netlink
messages for other clients in non init_net net namespaces.

Link: https://lore.kernel.org/r/20190723070205.6247-1-leon@kernel.org


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