RDMA/core: Add module param to disable device sharing among net ns

Add module parameter to change a sharing mode of ib_core early in the
boot process. This parameter helps to those systems where modern up
to date rdma tool (iproute2) package may not be available during
kernel upgrade cycle.

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