[IPV6]: create route6 proc init-fini functions

Make the proc creation/destruction to be a separate function. That
allows to remove the #ifdef CONFIG_PROC_FS in the init/fini function
and make them more readable.

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>