dlm: Constify struct configfs_item_operations and configfs_group_operations

dlm: 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, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  29436	  12952	    384	  42772	   a714	fs/dlm/config.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  30076	  12312	    384	  42772	   a714	fs/dlm/config.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>