[NETPOLL]: Set poll_owner to -1 before unlocking in netpoll_poll_unlock()

This trivial patch moves the assignment of poll_owner to -1 inside of
the lock.  This fixes a potential SMP race in the code.
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netpoll.h

@@ -53,8 +53,8 @@ static inline void netpoll_poll_lock(struct net_device *dev)
 static inline void netpoll_poll_unlock(struct net_device *dev)
 {
 	if (dev->np) {
-		spin_unlock(&dev->np->poll_lock);
 		dev->np->poll_owner = -1;
+		spin_unlock(&dev->np->poll_lock);
 	}
 }