qeth: suspicious rcu_dereference_check in recovery

qeth layer3 recovery invokes its set_multicast_list function, which
invokes function __vlan_find_dev_deep requiring rcu_read_lock or
rtnl lock. This causes kernel messages:

kernel: [ INFO: suspicious rcu_dereference_check() usage. ]
kernel: ---------------------------------------------------
kernel: net/8021q/vlan_core.c:70 invoked rcu_dereference_check() without protection!

kernel: stack backtrace:
kernel: CPU: 0 Not tainted 3.1.0 #9
kernel: Process qeth_recover (pid: 2078, task: 000000007e584680, ksp: 000000007e3e3930)
kernel: 000000007e3e3d08 000000007e3e3c88 0000000000000002 0000000000000000
kernel:       000000007e3e3d28 000000007e3e3ca0 000000007e3e3ca0 00000000005e77ce
kernel:       0000000000000000 0000000000000001 ffffffffffffffff 0000000000000001
kernel:       000000000000000d 000000000000000c 000000007e3e3cf0 0000000000000000
kernel:       0000000000000000 0000000000100a18 000000007e3e3c88 000000007e3e3cc8
kernel: Call Trace:
kernel: ([<0000000000100926>] show_trace+0xee/0x144)
kernel: [<00000000005d395c>] __vlan_find_dev_deep+0xb0/0x108
kernel: [<00000000004acd3a>] qeth_l3_set_multicast_list+0x976/0xe38
kernel: [<00000000004ae0f4>] __qeth_l3_set_online+0x75c/0x1498
kernel: [<00000000004aefec>] qeth_l3_recover+0xc4/0x1d0
kernel: [<0000000000185372>] kthread+0xa6/0xb0
kernel: [<00000000005ed4c6>] kernel_thread_starter+0x6/0xc
kernel: [<00000000005ed4c0>] kernel_thread_starter+0x0/0xc

The patch makes sure the rtnl lock is held once qeth recovery invokes
its set_multicast_list function.
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/s390/net/qeth_l3_main.c

@@ -3492,14 +3492,13 @@ static int __qeth_l3_set_online(struct ccwgroup_device *gdev, int recovery_mode)
 	else
 		netif_carrier_off(card->dev);
 	if (recover_flag == CARD_STATE_RECOVER) {
+		rtnl_lock();
 		if (recovery_mode)
 			__qeth_l3_open(card->dev);
-		else {
-			rtnl_lock();
+		else
 			dev_open(card->dev);
-			rtnl_unlock();
-		}
 		qeth_l3_set_multicast_list(card->dev);
+		rtnl_unlock();
 	}
 	/* let user_space know that device is online */
 	kobject_uevent(&gdev->dev.kobj, KOBJ_CHANGE);