net: Convert pg_net_ops

These pernet_operations create per-net pktgen threads
and /proc entries. These pernet subsys looks closed
in itself, and there are no pernet_operations outside
this file, which are interested in the threads.
Init and/or exit methods look safe to be executed
in parallel.

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