samples: configfs: Constify struct configfs_item_operations and configfs_group_operations

samples: configfs: 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
   7507	   6504	     64	  14075	   36fb	samples/configfs/configfs_sample.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   7827	   6184	     64	  14075	   36fb	samples/configfs/configfs_sample.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/70c5fd68fcc4d3ba1f18002012fae19acf4ce50b.1767007414.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>