net: Convert hwsim_net_ops

These pernet_operations allocate and destroy IDA identifier,
and these actions are synchronized by IDA subsystem locks.
Exit method removes mac80211_hwsim_data enteries from the lists,
and this is synchronized by hwsim_radio_lock with the rest
parallel pernet_operations. Also it queues destroy_radio()
work, and these work already may be executed in parallel
with any pernet_operations (as it's a work :). So, we may
mark these pernet_operations as async.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>