[IPV6]: Convert /proc/net/ipv6_route to seq_file interface

This removes last proc_net_create() user. Kudos to Benjamin Thery and
Stephen Hemminger for comments on previous version.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>