apparmor: add custom apparmorfs that will be used by policy namespace files

AppArmor policy needs to be able to be resolved based on the policy
namespace a task is confined by. Add a base apparmorfs filesystem that
(like nsfs) will exist as a kern mount and be accessed via jump_link
through a securityfs file.

Setup the base apparmorfs fns and data, but don't use it yet.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Reviewed-by: Seth Arnold <seth.arnold@canonical.com>
Reviewed-by: Kees Cook <keescook@chromium.org>