ipv6: Avoid rt6_probe() taking writer lock in the fast path

The patch checks neigh->nud_state before acquiring the writer lock.
Note that rt6_probe() is only used in CONFIG_IPV6_ROUTER_PREF.

40 udpflood processes and a /64 gateway route are used.
The gateway has NUD_PERMANENT.  Each of them is run for 30s.
At the end, the total number of finished sendto():

Before: 55M
After: 95M
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
CC: Julian Anastasov <ja@ssi.bg>
CC: YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -560,6 +560,9 @@ static void rt6_probe(struct rt6_info *rt)
 	rcu_read_lock_bh();
 	neigh = __ipv6_neigh_lookup_noref(rt->dst.dev, &rt->rt6i_gateway);
 	if (neigh) {
+		if (neigh->nud_state & NUD_VALID)
+			goto out;
+
 		work = NULL;
 		write_lock(&neigh->lock);
 		if (!(neigh->nud_state & NUD_VALID) &&
@@ -583,6 +586,7 @@ static void rt6_probe(struct rt6_info *rt)
 		schedule_work(&work->work);
 	}
 
+out:
 	rcu_read_unlock_bh();
 }
 #else