Currently when we specify AF_INET6 when it is disabled, we will get
all routes.

For example, we can boot kernel with ipv6.disable=1 and try to get ipv6
routes:
$ ip -6 route show
default via 192.168.122.1 dev eth0  proto static  metric 100
192.168.122.0/24 dev eth0  proto kernel  scope link  src 192.168.122.141  metric 100

Here are ipv4 routes and this is unexpected behaviour.
Signed-off-by: Andrew Vagin <avagin@openvz.org>