LSM: Avoid warnings about potentially unused hook variables

Building with W=1 shows many unused const variable warnings. These can
be silenced, as we're well aware of their being potentially unused:

./include/linux/lsm_hook_defs.h:36:18: error: 'ptrace_access_check_default' defined but not used [-Werror=unused-const-variable=]
   36 | LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child,
      |                  ^~~~~~~~~~~~~~~~~~~
security/security.c:706:32: note: in definition of macro 'LSM_RET_DEFAULT'
  706 | #define LSM_RET_DEFAULT(NAME) (NAME##_default)
      |                                ^~~~
security/security.c:711:9: note: in expansion of macro 'DECLARE_LSM_RET_DEFAULT_int'
  711 |         DECLARE_LSM_RET_DEFAULT_##RET(DEFAULT, NAME)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/lsm_hook_defs.h:36:1: note: in expansion of macro 'LSM_HOOK'
   36 | LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child,
      | ^~~~~~~~

Cc: James Morris <[email protected]>
Cc: "Serge E. Hallyn" <[email protected]>
Cc: Paul Moore <[email protected]>
Cc: Casey Schaufler <[email protected]>
Cc: KP Singh <[email protected]>
Cc: [email protected]
Reported-by: kernel test robot <[email protected]>
Link: https://lore.kernel.org/linux-mm/[email protected]/
Fixes: 98e828a ("security: Refactor declaration of LSM hooks")
Signed-off-by: Kees Cook <[email protected]>
Acked-by: James Morris <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

security/security.c

@@ -706,7 +706,7 @@ static int lsm_superblock_alloc(struct super_block *sb)
 #define LSM_RET_DEFAULT(NAME) (NAME##_default)
 #define DECLARE_LSM_RET_DEFAULT_void(DEFAULT, NAME)
 #define DECLARE_LSM_RET_DEFAULT_int(DEFAULT, NAME) \
-	static const int LSM_RET_DEFAULT(NAME) = (DEFAULT);
+	static const int __maybe_unused LSM_RET_DEFAULT(NAME) = (DEFAULT);
 #define LSM_HOOK(RET, DEFAULT, NAME, ...) \
 	DECLARE_LSM_RET_DEFAULT_##RET(DEFAULT, NAME)