selinux: Deprecate /sys/fs/selinux/user

The only known user of this interface was libselinux and its
internal usage of this interface for get_ordered_context_list(3)
was removed in Feb 2020, with a deprecation warning added to
security_compute_user(3) at the same time. Add a deprecation
warning to the kernel and schedule it for final removal in 2025.

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>