null_blk: Constify struct configfs_item_operations and configfs_group_operations

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 <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jens Axboe <axboe@kernel.dk>