netpoll: fix a missing dev refcounting

__dev_get_by_name() doesn't refcount the network device,
so we have to do this by ourselves. Noticed by Eric.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -1056,6 +1056,7 @@ int netpoll_setup(struct netpoll *np)
 		err = -ENODEV;
 		goto unlock;
 	}
+	dev_hold(ndev);
 
 	if (netdev_master_upper_dev_get(ndev)) {
 		np_err(np, "%s is a slave device, aborting\n", np->dev_name);