Register netdevice notifier for every iptable entry
is not good, since this breaks modularity, and
the hidden synchronization is based on rtnl_lock().

This patch reworks the synchronization via new lock,
while the rest of logic remains as it was before.
This is required for the next patch.

Tested via:

while :; do
	unshare -n iptables -t mangle -A OUTPUT -j TEE --gateway 1.1.1.2 --oif lo;
done
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>