net: Convert cfg80211_pernet_ops

This patch finishes converting pernet_operations
registered in net/wireless directory.

These pernet_operations have only exit method,
which moves devices to init_net. This action
is not pernet_operations-specific, and function
cfg80211_switch_netns() may be called all time
during the system life. All necessary protection
against concurrent cfg80211_pernet_exit() is made
by rtnl_lock(). So, cfg80211_pernet_ops is able
to be marked as async.

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