landlock: Enforce capability restrictions

Add Landlock enforcement for capability use via the LSM capable hook.
This lets a sandboxed process restrict which Linux capabilities it can
exercise, using LANDLOCK_PERMISSION_CAPABILITY_USE and per-capability
rules.

The check is a flat per-layer allowed-capabilities test, with no
domain-ancestry bypass, no cross-namespace discriminant, and no
dependency on the target user namespace.  These categorical denials
return -EPERM, like the namespace permission introduced by the previous
commit, and they mirror its per-capability allowed and quiet masks, so
LANDLOCK_ADD_RULE_QUIET stays rejected for this rule type.  Successful
capability rules and denials are traced as the previous commit
describes.

Enforce only at capability exercise time rather than modifying the
credential's capability sets.  Modifying them would give capget(2) an
accurate view of usable capabilities, but no LSM other than commoncap
does so; Landlock follows that convention.  A sandboxed process inside a
user namespace therefore sees all capabilities via capget(2) but
receives -EPERM when attempting to use one that is denied.

An earlier design bypassed this hook for namespace management through a
domain-ancestry comparison with the namespace creator's stored Landlock
domain.  That made a Landlock decision depend on kernel namespace state,
and the ns != cred->user_ns heuristic did not accurately identify
namespace-management operations.  The flat, namespace-agnostic check
instead enforces capability and namespace restrictions independently;
creating a non-user namespace requires an allowed CAP_SYS_ADMIN even
when combined with CLONE_NEWUSER in one unshare().

Cc: Christian Brauner <brauner@kernel.org>
Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Serge E. Hallyn <serge@hallyn.com>
Link: https://patch.msgid.link/20261007100255.1333386-5-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

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