RDMA/core: Add command to set ib_core device net namspace sharing mode

Add netlink command that enables/disables sharing rdma device among
multiple net namespaces.

Using rdma tool,
$rdma sys set netns shared (default mode)

When rdma subsystem netns mode is set to shared mode, rdma devices
will be accessible in all net namespaces.

Using rdma tool,
$rdma sys set netns exclusive

When rdma subsystem netns mode is set to exclusive mode, devices
will be accessible in only one net namespace at any given
point of time.

If there are any net namespaces other than default init_net exists,
while executing this command, it will fail and mode cannot be changed.

To change this mode, netlink command is used instead of sysctl, because
netlink command allows to auto load a module.

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