md-cluster: Constify struct md_cluster_operations

md-cluster: Constify struct md_cluster_operations

'struct md_cluster_operations' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  51941	   1442	     80	  53463	   d0d7	drivers/md/md-cluster.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  52133	   1246	     80	  53459	   d0d3	drivers/md/md-cluster.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/3727f3ce9693cae4e62ae6778ea13971df805479.1719173852.git.christophe.jaillet@wanadoo.fr