RDMA/core: make ib_device.add method optional

ib_clients can indeed fill .add to NULL, but then they will not see
any device removal notifications. The reason is that that
ib_register_client and ib_register_device checked existence of .add
before adding the creating a corresponding client_data and adding
it to the list. Simple condition reverse fixes the issue.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/core/device.c

@@ -489,7 +489,7 @@ int ib_register_device(struct ib_device *device,
 	device->reg_state = IB_DEV_REGISTERED;
 
 	list_for_each_entry(client, &client_list, list)
-		if (client->add && !add_client_context(device, client))
+		if (!add_client_context(device, client) && client->add)
 			client->add(device);
 
 	down_write(&lists_rwsem);
@@ -577,7 +577,7 @@ int ib_register_client(struct ib_client *client)
 	mutex_lock(&device_mutex);
 
 	list_for_each_entry(device, &device_list, core_list)
-		if (client->add && !add_client_context(device, client))
+		if (!add_client_context(device, client) && client->add)
 			client->add(device);
 
 	down_write(&lists_rwsem);