landlock: Log the TGID of the domain creator

As for other Audit's "pid" fields, Landlock should use the task's TGID
instead of its TID.  Fix this issue by keeping a reference to the TGID
of the domain creator.

Existing tests already check for the PID but only with the thread group
leader, so always the TGID.  A following patch adds dedicated tests for
non-leader thread.

Remove the current_real_cred() check which does not make sense because
we only reference a struct pid, whereas a previous version did reference
a struct cred instead.

Cc: Christian Brauner <brauner@kernel.org>
Cc: Paul Moore <paul@paul-moore.com>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20250410171725.1265860-1-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>