netns: fix ip_rt_frag_needed rt_is_expired

Running recent kernels, and using a particular vpn gateway, I've been
having to edit my mails down to get them accepted by the smtp server.

Git bisect led to commit e84f84f2 -
netns: place rt_genid into struct net.  The conversion from a != test
to rt_is_expired() put one negative too many: and now my mail works.
Signed-off-by: NHugh Dickins <hugh@veritas.com>
Acked-by: NDenis V. Lunev <den@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1502,7 +1502,7 @@ unsigned short ip_rt_frag_needed(struct net *net, struct iphdr *iph,
 				    rth->fl.iif != 0 ||
 				    dst_metric_locked(&rth->u.dst, RTAX_MTU) ||
 				    !net_eq(dev_net(rth->u.dst.dev), net) ||
-				    !rt_is_expired(rth))
+				    rt_is_expired(rth))
 					continue;
 
 				if (new_mtu < 68 || new_mtu >= old_mtu) {