ipv4: Fix flushing of cached routing informations

Currently we can not flush cached pmtu/redirect informations via
the ipv4_sysctl_rtcache_flush sysctl. We need to check the rt_genid
of the old route and reset the nh exeption if the old route is
expired when we bind a new route to a nh exeption.
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1163,8 +1163,12 @@ static bool rt_bind_exception(struct rtable *rt, struct fib_nh_exception *fnhe,
 	spin_lock_bh(&fnhe_lock);
 
 	if (daddr == fnhe->fnhe_daddr) {
-		struct rtable *orig;
-
+		struct rtable *orig = rcu_dereference(fnhe->fnhe_rth);
+		if (orig && rt_is_expired(orig)) {
+			fnhe->fnhe_gw = 0;
+			fnhe->fnhe_pmtu = 0;
+			fnhe->fnhe_expires = 0;
+		}
 		if (fnhe->fnhe_pmtu) {
 			unsigned long expires = fnhe->fnhe_expires;
 			unsigned long diff = expires - jiffies;
@@ -1181,7 +1185,6 @@ static bool rt_bind_exception(struct rtable *rt, struct fib_nh_exception *fnhe,
 		} else if (!rt->rt_gateway)
 			rt->rt_gateway = daddr;
 
-		orig = rcu_dereference(fnhe->fnhe_rth);
 		rcu_assign_pointer(fnhe->fnhe_rth, rt);
 		if (orig)
 			rt_free(orig);