[IPV4]: rt_cache_get_next should take rt_genid into account.

In the other case /proc/net/rt_cache will look inconsistent in respect to
genid.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>