[IPV4/IPV6/DECNET]: Small cleanup for fib rules.

This patch slightly cleanups FIB rules framework. rules_list as a pointer
on struct fib_rules_ops is useless. It is always assigned with a static
per/subsystem list in IPv4, IPv6 and DecNet.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>