[NETPOLL]: Fix a leak-n-bug in netpoll_cleanup()

93ec2c723e3f8a216dde2899aeb85c648672bc6b applied excessive duct tape to
the netpoll beast's netpoll_cleanup(), thus substituting one leak with
another, and opening up a little buglet :-)

net_device->npinfo (netpoll_info) is a shared and refcounted object and
cannot simply be set NULL the first time netpoll_cleanup() is called.
Otherwise, further netpoll_cleanup()'s see np->dev->npinfo == NULL and
become no-ops, thus leaking. And it's a bug too: the first call to
netpoll_cleanup() would thus (annoyingly) "disable" other (still alive)
netpolls too. Maybe nobody noticed this because netconsole (only user
of netpoll) never supported multiple netpoll objects earlier.

This is a trivial and obvious one-line fixlet.

Signed-off-by: Satyam Sharma <ssatyam@cse.iitk.ac.in>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -783,7 +783,6 @@ void netpoll_cleanup(struct netpoll *np)
 				spin_unlock_irqrestore(&npinfo->rx_lock, flags);
 			}
 
-			np->dev->npinfo = NULL;
 			if (atomic_dec_and_test(&npinfo->refcnt)) {
 				skb_queue_purge(&npinfo->arp_tx);
 				skb_queue_purge(&npinfo->txq);
@@ -796,6 +795,7 @@ void netpoll_cleanup(struct netpoll *np)
 					kfree_skb(skb);
 				}
 				kfree(npinfo);
+				np->dev->npinfo = NULL;
 			}
 		}