landlock: Optimize stack usage when !CONFIG_AUDIT

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 <gnoack@google.com>
Reported-by: Tingmao Wang <m@maowtm.org>
Closes: https://lore.kernel.org/r/eb86863b-53b0-460b-b223-84dd31d765b9@maowtm.org
Fixes: 2fc80c69df82 ("landlock: Log file-related denials")
Link: https://lore.kernel.org/r/20251219142302.744917-2-mic@digikod.net
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
[mic: Improve stack usage measurement accuracy with scripts/stackdelta]
Signed-off-by: Mickaël Salaün <mic@digikod.net>