net: Convert sit_net_ops

net: Convert sit_net_ops

These pernet_operations are similar to ip6_tnl_net_ops. Exit method
unregisters all net sit devices, and it looks like another
pernet_operations are not interested in foreign net sit list.
Init method registers netdevice. So, it's possible to mark them async.

Signed-off-by: Kirill Tkhai <[email protected]>
Signed-off-by: David S. Miller <[email protected]>