[IPV6]: route6 remove ifdef for fib_rules

The patch defines the usual static inline functions when the code is
disabled for fib6_rules. That's allow to remove some ifdef in route.c
file and make the code a little more clear.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>