landlock: Use scoped guards for ruleset

Simplify error handling by replacing goto statements with automatic
calls to landlock_put_ruleset() when going out of scope.

This change will be easy to backport to v6.6 if needed, only the
kernel.h include line conflicts.  As for any other similar changes, we
should be careful when backporting without goto statements.

Add missing include file.

Reviewed-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250113161112.452505-2-mic@digikod.net


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