rdma: Add an option to query,set net namespace sharing sys parameter

rdma: Add an option to query,set net namespace sharing sys parameter

Enrich rdmatool with an option to query rdma subsystem parameter
whether rdma devices are shared among multiple network namespaces
or exclusive to single network namespace.

rdma tool command examples and output.

$ rdma system show
netns shared

$ rdma system set netns exclusive

$ rdma system show
netns exclusive

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