route: filter routes by family if it's specified

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>

ip/iproute.c

@@ -145,6 +145,9 @@ static int filter_nlmsg(struct nlmsghdr *n, struct rtattr **tb, int host_len)
 
 	table = rtm_get_table(r, tb);
 
+	if (preferred_family != AF_UNSPEC && r->rtm_family != preferred_family)
+		return 0;
+
 	if (r->rtm_family == AF_INET6 && table != RT_TABLE_MAIN)
 		ip6_multiple_tables = 1;