ntb: Constify struct bus_type

'struct bus_type' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  69682	   4593	    152	  74427	  122bb	drivers/ntb/ntb_transport.o
   5847	    448	     32	   6327	   18b7	drivers/ntb/core.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  69858	   4433	    152	  74443	  122cb	drivers/ntb/ntb_transport.o
   6007	    288	     32	   6327	   18b7	drivers/ntb/core.o

Signed-off-by: Christophe JAILLET <[email protected]>
Reviewed-by: Dave Jiang <[email protected]>
Signed-off-by: Jon Mason <[email protected]>

Author: tititiou36

drivers/ntb/core.c

@@ -72,7 +72,7 @@ MODULE_VERSION(DRIVER_VERSION);
 MODULE_AUTHOR(DRIVER_AUTHOR);
 MODULE_DESCRIPTION(DRIVER_DESCRIPTION);
 
-static struct bus_type ntb_bus;
+static const struct bus_type ntb_bus;
 static void ntb_dev_release(struct device *dev);
 
 int __ntb_register_client(struct ntb_client *client, struct module *mod,
@@ -298,7 +298,7 @@ static void ntb_dev_release(struct device *dev)
 	complete(&ntb->released);
 }
 
-static struct bus_type ntb_bus = {
+static const struct bus_type ntb_bus = {
 	.name = "ntb",
 	.probe = ntb_probe,
 	.remove = ntb_remove,

drivers/ntb/ntb_transport.c

@@ -314,7 +314,7 @@ static void ntb_transport_bus_remove(struct device *dev)
 	put_device(dev);
 }
 
-static struct bus_type ntb_transport_bus = {
+static const struct bus_type ntb_transport_bus = {
 	.name = "ntb_transport",
 	.match = ntb_transport_bus_match,
 	.probe = ntb_transport_bus_probe,