"struct ocfs2_stack_operations" are not modified in this driver.

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

In order to do it, "struct ocfs2_stack_plugin" also needs to be adjusted
to this new const qualifier.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   6241	    644	      0	   6885	   1ae5	fs/ocfs2/stack_o2cb.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   6337	    548	      0	   6885	   1ae5	fs/ocfs2/stack_o2cb.o

Link: https://lkml.kernel.org/r/f52dab89ee0049ec6271de29183a781efbb275ab.1718398605.git.christophe.jaillet@wanadoo.frSigned-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Joseph Qi <jiangqi903@gmail.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Gang He <ghe@suse.com>
Cc: Jun Piao <piaojun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>