net/dst: dst_dev_event() called after other notifiers

Followup of commit ef885afb (net: use rcu_barrier() in
rollback_registered_many)

dst_dev_event() scans a garbage dst list that might be feeded by various
network notifiers at device dismantle time.

Its important to call dst_dev_event() after other notifiers, or we might
enter the infamous msleep(250) in netdev_wait_allrefs(), and wait one
second before calling again call_netdevice_notifiers(NETDEV_UNREGISTER,
dev) to properly remove last device references.

Use priority -10 to let dst_dev_notifier be called after other network
notifiers (they have the default 0 priority)
Reported-by: NBen Greear <greearb@candelatech.com>
Reported-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Reported-by: NOctavian Purdila <opurdila@ixiacom.com>
Reported-by: NBenjamin LaHaise <bcrl@kvack.org>
Tested-by: NBen Greear <greearb@candelatech.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dst.c

@@ -370,6 +370,7 @@ static int dst_dev_event(struct notifier_block *this, unsigned long event,
 
 static struct notifier_block dst_dev_notifier = {
 	.notifier_call	= dst_dev_event,
+	.priority = -10, /* must be called after other network notifiers */
 };
 
 void __init dst_init(void)