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 <[e-mail]>
Link: https://lore.kernel.org/r/f83cd8469c[e-mail]
Reviewed-by: Bodo Stroesser <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/target/target_core_user.c

···
 	[TCMU_MCGRP_CONFIG] = { .name = "config", },
 };
 
-static struct nla_policy tcmu_attr_policy[TCMU_ATTR_MAX+1] = {
+static const struct nla_policy tcmu_attr_policy[TCMU_ATTR_MAX + 1] = {
 	[TCMU_ATTR_DEVICE]	= { .type = NLA_STRING },
 	[TCMU_ATTR_MINOR]	= { .type = NLA_U32 },
 	[TCMU_ATTR_CMD_STATUS]	= { .type = NLA_S32 },
···
 	Opt_cmd_ring_size_mb, Opt_err,
 };
 
-static match_table_t tokens = {
+static const match_table_t tokens = {
 	{Opt_dev_config, "dev_config=%s"},
 	{Opt_dev_size, "dev_size=%s"},
 	{Opt_hw_block_size, "hw_block_size=%d"},