netconsole: fix a deadlock with rtnl and netconsole's mutex

This bug was introduced by commit
7a163bfb ("netconsole: avoid a crash with
multiple sysfs writers"). In store_enabled() we have the following
sequence: acquire nt->mutex then rtnl, but in the netconsole netdev
notifier we have rtnl then nt->mutex effectively leading to a deadlock.
The NULL pointer dereference that the above commit tries to fix is
actually due to another bug in netpoll_cleanup(). This is fixed by dropping
the mutex from the netdev notifier as it's already protected by rtnl.
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/netconsole.c

@@ -684,15 +684,12 @@ static int netconsole_netdev_event(struct notifier_block *this,
 			case NETDEV_RELEASE:
 			case NETDEV_JOIN:
 			case NETDEV_UNREGISTER:
-				/*
-				 * rtnl_lock already held
+				/* rtnl_lock already held
 				 * we might sleep in __netpoll_cleanup()
 				 */
 				spin_unlock_irqrestore(&target_list_lock, flags);
 
-				mutex_lock(&nt->mutex);
 				__netpoll_cleanup(&nt->np);
-				mutex_unlock(&nt->mutex);
 
 				spin_lock_irqsave(&target_list_lock, flags);
 				dev_put(nt->np.dev);