net: Convert br_net_ops

These pernet_operations are similar to bond_net_ops. Exit method
unregisters all net bridge devices, and it looks like another
pernet_operations are not interested in foreign net bridge list.
So, it's possible to mark them async.
Signed-off-by: NKirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br.c

@@ -188,6 +188,7 @@ static void __net_exit br_net_exit(struct net *net)
 
 static struct pernet_operations br_net_ops = {
 	.exit	= br_net_exit,
+	.async	= true,
 };
 
 static const struct stp_proto br_stp_proto = {