net: use rcu_barrier() in rollback_registered_many

netdev_wait_allrefs() waits that all references to a device vanishes.

It currently uses a _very_ pessimistic 250 ms delay between each probe.
Some users reported that no more than 4 devices can be dismantled per
second, this is a pretty serious problem for some setups.

Most of the time, a refcount is about to be released by an RCU callback,
that is still in flight because rollback_registered_many() uses a
synchronize_rcu() call instead of rcu_barrier(). Problem is visible if
number of online cpus is one, because synchronize_rcu() is then a no op.

time to remove 50 ipip tunnels on a UP machine :

before patch : real 11.910s
after patch : real 1.250s

Reported-by: Nicolas Dichtel <[e-mail]>
Reported-by: Octavian Purdila <[e-mail]>
Reported-by: Benjamin LaHaise <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/dev.c

···
 	dev = list_first_entry(head, struct net_device, unreg_list);
 	call_netdevice_notifiers(NETDEV_UNREGISTER_BATCH, dev);
 
-	synchronize_net();
+	rcu_barrier();
 
 	list_for_each_entry(dev, head, unreg_list)
 		dev_put(dev);