ndisc: Do not try to update "updated" time if neighbour has already gone.

Commit 2152caea ("ipv6: Do not depend on rt->n in rt6_probe().")
introduce a bug to try to update "updated" time in neighbour
structure.
Update the "updated" time only if neighbour is available.

Bug was found by Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -492,10 +492,10 @@ static void rt6_probe(struct rt6_info *rt)
 		struct in6_addr mcaddr;
 		struct in6_addr *target;
 
-		neigh->updated = jiffies;
-
-		if (neigh)
+		if (neigh) {
+			neigh->updated = jiffies;
 			write_unlock(&neigh->lock);
+		}
 
 		target = (struct in6_addr *)&rt->rt6i_gateway;
 		addrconf_addr_solict_mult(target, &mcaddr);