net: netpoll: fix incorrect refcount handling causing incorrect cleanup

commit efa95b01da18 ("netpoll: fix use after free") incorrectly
ignored the refcount and prematurely set dev->npinfo to NULL during
netpoll cleanup, leading to improper behavior and memory leaks.

Scenario causing lack of proper cleanup:

1) A netpoll is associated with a NIC (e.g., eth0) and netdev->npinfo is
allocated, and refcnt = 1
- Keep in mind that npinfo is shared among all netpoll instances. In
this case, there is just one.

2) Another netpoll is also associated with the same NIC and
npinfo->refcnt += 1.
- Now dev->npinfo->refcnt = 2;
- There is just one npinfo associated to the netdev.

3) When the first netpolls goes to clean up:
- The first cleanup succeeds and clears np->dev->npinfo, ignoring
refcnt.
- It basically calls `RCU_INIT_POINTER(np->dev->npinfo, NULL);`
- Set dev->npinfo = NULL, without proper cleanup
- No ->ndo_netpoll_cleanup() is either called

4) Now the second target tries to clean up
- The second cleanup fails because np->dev->npinfo is already NULL.
* In this case, ops->ndo_netpoll_cleanup() was never called, and
the skb pool is not cleaned as well (for the second netpoll
instance)
- This leaks npinfo and skbpool skbs, which is clearly reported by
kmemleak.

Revert commit efa95b01da18 ("netpoll: fix use after free") and adds
clarifying comments emphasizing that npinfo cleanup should only happen
once the refcount reaches zero, ensuring stable and correct netpoll
behavior.

Cc: <[e-mail]> # 3.17.x
Cc: Jay Vosburgh <[e-mail]>
Fixes: efa95b01da18 ("netpoll: fix use after free")
Signed-off-by: Breno Leitao <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/netpoll.c

···
 	if (!npinfo)
 		return;
 
+	/* At this point, there is a single npinfo instance per netdevice, and
+	 * its refcnt tracks how many netpoll structures are linked to it. We
+	 * only perform npinfo cleanup when the refcnt decrements to zero.
+	 */
 	if (refcount_dec_and_test(&npinfo->refcnt)) {
 		const struct net_device_ops *ops;
 
···
 
 		RCU_INIT_POINTER(np->dev->npinfo, NULL);
 		call_rcu(&npinfo->rcu, rcu_cleanup_netpoll_info);
-	} else
-		RCU_INIT_POINTER(np->dev->npinfo, NULL);
+	}
 
 	skb_pool_flush(np);
 }