net: Convert log pernet_operations

These pernet_operations use nf_log_set() and nf_log_unset()
in their methods:

	nf_log_bridge_net_ops
	nf_log_arp_net_ops
	nf_log_ipv4_net_ops
	nf_log_ipv6_net_ops
	nf_log_netdev_net_ops

Nobody can send such a packet to a net before it's became
registered, nobody can send a packet after all netdevices
are unregistered. So, these pernet_operations are able
to be marked as async.

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