genetlink: use idr_alloc_cyclic for family->id assignment

When allocating the next family->id it makes more sense to use
idr_alloc_cyclic to avoid re-using a previously used family->id as much
as possible.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>