landlock: Optimize stack usage when !CONFIG_AUDIT

Until now, each landlock_request struct were allocated on the stack, even
if not really used, because is_access_to_paths_allowed() unconditionally
modified the passed references. Even if the changed landlock_request
variables are not used, the compiler is not smart enough to detect this
case.

To avoid this issue, explicitly disable the related code when
CONFIG_AUDIT is not set, which enables elision of log_request_parent*
and associated caller's stack variables thanks to dead code elimination.
This makes it possible to reduce the stack frame by 32 bytes for the
path_link and path_rename hooks, and by 20 bytes for most other
filesystem hooks.

Here is a summary of scripts/stackdelta before and after this change
when CONFIG_AUDIT is disabled:

current_check_refer_path 560 320 -240
current_check_access_path 328 184 -144
hook_file_open 328 184 -144
is_access_to_paths_allowed 376 360 -16

Also, add extra pointer checks to be more future-proof.

Cc: Günther Noack <[e-mail]>
Reported-by: Tingmao Wang <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]
Fixes: 2fc80c69df82 ("landlock: Log file-related denials")
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Günther Noack <[e-mail]>
[mic: Improve stack usage measurement accuracy with scripts/stackdelta]
Signed-off-by: Mickaël Salaün <[e-mail]>

security/landlock/fs.c

···
 	}
 	path_put(&walker_path);
 
-	if (!allowed_parent1) {
+	/*
+	 * Check CONFIG_AUDIT to enable elision of log_request_parent* and
+	 * associated caller's stack variables thanks to dead code elimination.
+	 */
+#ifdef CONFIG_AUDIT
+	if (!allowed_parent1 && log_request_parent1) {
 		log_request_parent1->type = LANDLOCK_REQUEST_FS_ACCESS;
 		log_request_parent1->audit.type = LSM_AUDIT_DATA_PATH;
 		log_request_parent1->audit.u.path = *path;
···
 			ARRAY_SIZE(*layer_masks_parent1);
 	}
 
-	if (!allowed_parent2) {
+	if (!allowed_parent2 && log_request_parent2) {
 		log_request_parent2->type = LANDLOCK_REQUEST_FS_ACCESS;
 		log_request_parent2->audit.type = LSM_AUDIT_DATA_PATH;
 		log_request_parent2->audit.u.path = *path;
···
 		log_request_parent2->layer_masks_size =
 			ARRAY_SIZE(*layer_masks_parent2);
 	}
+#endif /* CONFIG_AUDIT */
+
 	return allowed_parent1 && allowed_parent2;
 }