net: Convert wext_pernet_ops

These pernet_operations initialize and purge net::wext_nlevents
queue, and are not touched by foreign pernet_operations.

Mark them async.

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