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>