scsi: target: core: Constify struct target_opcode_descriptor

Instances of 'struct target_opcode_descriptor' are not modified in this
driver.

Constifying this structure moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  53602	  19750	      0	  73352	  11e88	drivers/target/target_core_spc.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  58594	  14758	      0	  73352	  11e88	drivers/target/target_core_spc.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/889ee46e75db33e8ab997a627a1d3d651ad648db.1747592774.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: tititiou36