Extend ib_device_get_by_index() API to check device access for
net namespace for serving netlink commands.

Also enforce net ns check on dumpit commands which iterate over all
registered rdma devices and which don't call ib_device_get_by_index().
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>