netpoll: fix use after free

After a bonding master reclaims the netpoll info struct, slaves could
still hold a pointer to the reclaimed data. This patch fixes it: as
soon as netpoll_async_cleanup is called for a slave (eg. when
un-enslaved), we make sure that this slave doesn't point to the data.
Signed-off-by: NDavid Decotigny <decot@googlers.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -822,7 +822,8 @@ void __netpoll_cleanup(struct netpoll *np)
 
 		RCU_INIT_POINTER(np->dev->npinfo, NULL);
 		call_rcu_bh(&npinfo->rcu, rcu_cleanup_netpoll_info);
-	}
+	} else
+		RCU_INIT_POINTER(np->dev->npinfo, NULL);
 }
 EXPORT_SYMBOL_GPL(__netpoll_cleanup);