net: fix rcu use in ip_route_output_slow

__in_dev_get_rtnl(dev_out) is called while RTNL is not held, thus
triggers a lockdep fault.

At this point, we only perform a raw test of dev_out->ip_ptr being NULL,
we dont need to make sure ip_ptr cant changed right after.

We can use rcu_dereference_raw() for this.
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -2579,7 +2579,7 @@ static int ip_route_output_slow(struct net *net, struct rtable **rp,
 			goto out;
 
 		/* RACE: Check return value of inet_select_addr instead. */
-		if (__in_dev_get_rtnl(dev_out) == NULL) {
+		if (rcu_dereference_raw(dev_out->ip_ptr) == NULL) {
 			dev_put(dev_out);
 			goto out;	/* Wrong error code */
 		}