null_blk: Constify struct configfs_item_operations and configfs_group_operations

'struct configfs_item_operations' and 'configfs_group_operations' are not
modified in this driver.

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

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
 100263	  37808	   2752	 140823	  22617	drivers/block/null_blk/main.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
 100423	  37648	   2752	 140823	  22617	drivers/block/null_blk/main.o

Signed-off-by: Christophe JAILLET <[email protected]>

Author: tititiou36

drivers/block/null_blk/main.c

@@ -642,7 +642,7 @@ static void nullb_device_release(struct config_item *item)
 	null_free_dev(dev);
 }
 
-static struct configfs_item_operations nullb_device_ops = {
+static const struct configfs_item_operations nullb_device_ops = {
 	.release	= nullb_device_release,
 };
 
@@ -739,7 +739,7 @@ static struct configfs_attribute *nullb_group_attrs[] = {
 	NULL,
 };
 
-static struct configfs_group_operations nullb_group_ops = {
+static const struct configfs_group_operations nullb_group_ops = {
 	.make_group	= nullb_group_make_group,
 	.drop_item	= nullb_group_drop_item,
 };