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: David Decotigny <decot@googlers.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index e33937f..907fb5e 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -822,7 +822,8 @@
 
 		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);