net: Convert mpls_net_ops

These pernet_operations register and unregister sysctl table.
Exit methods frees platform_labels from net::mpls::platform_label.
Everything is per-net, and they looks safe to be marked async.

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