net: Convert ip_set_net_ops

These pernet_operations initialize and destroy
net_generic(net, ip_set_net_id)-related data.
Since ip_set is under CONFIG_IP_SET, it's easy
to watch drivers, which depend on this config.
All of them are in net/netfilter/ipset directory,
except of net/netfilter/xt_set.c. There are no
more drivers, which use ip_set, and all of
the above don't register another pernet_operations.
Also, there are is no indirect users, as header
file include/linux/netfilter/ipset/ip_set.h does
not define indirect users by something like this:

	#ifdef CONFIG_IP_SET
	extern func(void);
	#else
	static inline func(void);
	#endif

So, there are no more pernet operations, dereferencing
net_generic(net, ip_set_net_id).

ip_set_net_ops are OK to be executed in parallel
for several net, so we mark them as async.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>