[NETNS]: Namespacing IPv4 fib rules.

The final trick for rules: place fib4_rules_ops into struct net and
modify initialization path for this.

Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Acked-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>