net: Convert l2tp_net_ops

Init method is rather simple. Exit method queues del_work
for every tunnel from per-net list. This seems to be safe
to be marked async.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>