netfilter: conntrack: expedite rcu in nf_conntrack_cleanup_net_list

nf_conntrack_cleanup_net_list() is calling synchronize_net()
while RTNL is not held. This effectively calls synchronize_rcu().

synchronize_rcu() is much slower than synchronize_rcu_expedited(),
and cleanup_net() is currently single threaded. In many workloads
we want cleanup_net() to be faster, in order to free memory and various
sysfs and procfs entries as fast as possible.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Jozsef Kadlecsik <kadlec@netfilter.org>
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/nf_conntrack_core.c

@@ -2530,7 +2530,7 @@ void nf_conntrack_cleanup_net_list(struct list_head *net_exit_list)
 	 *  netfilter framework.  Roll on, two-stage module
 	 *  delete...
 	 */
-	synchronize_net();
+	synchronize_rcu_expedited();
 i_see_dead_people:
 	busy = 0;
 	list_for_each_entry(net, net_exit_list, exit_list) {