net: Add missing extra2 parameter for ip_default_ttl sysctl

Commit 76e6ebfb40a2455c18234dcb0f9df37533215461 ("netns: add namespace
parameter to rt_cache_flush") acceses the extra2 parameter of the
ip_default_ttl ctl_table, but it is never set to a meaningful
value. When e84f84f276473dcc673f360e8ff3203148bdf0e2 ("netns: place
rt_genid into struct net") is applied, we'll oops in
rt_cache_invalidate(). Set extra2 to init_net, to avoid that.

Reported-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Tested-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/sysctl_net_ipv4.c

@@ -232,6 +232,7 @@ static struct ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &ipv4_doint_and_flush,
 		.strategy	= &ipv4_doint_and_flush_strategy,
+		.extra2		= &init_net,
 	},
 	{
 		.ctl_name	= NET_IPV4_NO_PMTU_DISC,