scsi: target: tcmu: Constify some structures

'struct nla_policy' and 'struct match_table_t' are not modified in this
driver.

Constifying these structures moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers, which is the case of struct nla_policy.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  93188	   6933	    338	 100459	  1886b	drivers/target/target_core_user.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  93508	   6581	    338	 100427	  1884b	drivers/target/target_core_user.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/f83cd8469cc17391178e1181e8c26c4c1fb6028f.1731330634.git.christophe.jaillet@wanadoo.fr


Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>