NTB: Handle ntb client device probes without present hardware

Attempts to probe client ntb drivers without ntb hardware present will
result in null pointer dereference due to the lack of the ntb bus device
being registers.  Check to see if this is the case, and fail all calls
by the clients registering their drivers.

Signed-off-by: Jon Mason <jon.mason@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/ntb/ntb_transport.c

@@ -288,6 +288,9 @@ int ntb_register_client_dev(char *device_name)
 	struct ntb_transport *nt;
 	int rc;
 
+	if (list_empty(&ntb_transport_list))
+		return -ENODEV;
+
 	list_for_each_entry(nt, &ntb_transport_list, entry) {
 		struct device *dev;
 
@@ -336,6 +339,9 @@ int ntb_register_client(struct ntb_client *drv)
 {
 	drv->driver.bus = &ntb_bus_type;
 
+	if (list_empty(&ntb_transport_list))
+		return -ENODEV;
+
 	return driver_register(&drv->driver);
 }
 EXPORT_SYMBOL_GPL(ntb_register_client);