[UNIX]: Use ctl paths to register unix ctl tables

Unlike previous ones, this patch is useful by its own,
as it decreases the vmlinux size :)

But it will be used later, when the per-namespace sysctl
is added.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>