RDMA/cma: add missed unregister_pernet_subsys in init failure

The driver forgets to call unregister_pernet_subsys() in the error path
of cma_init().
Add the missed call to fix it.

Fixes: 4be74b42a6 ("IB/cma: Separate port allocation to network namespaces")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Link: https://lore.kernel.org/r/20191206012426.12744-1-hslester96@gmail.com
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/core/cma.c

@@ -4763,6 +4763,7 @@ err_ib:
 err:
 	unregister_netdevice_notifier(&cma_nb);
 	ib_sa_unregister_client(&sa_client);
+	unregister_pernet_subsys(&cma_pernet_operations);
 err_wq:
 	destroy_workqueue(cma_wq);
 	return ret;