net: skip genenerating uevents for network namespaces that are exiting

No one can see these events, because a network namespace can not be
destroyed, if it has sockets.

Unlike other devices, uevent-s for network devices are generated
only inside their network namespaces. They are filtered in
kobj_bcast_filter()

My experiments shows that net namespaces are destroyed more 30% faster
with this optimization.

Here is a perf output for destroying network namespaces without this
patch.

-   94.76%     0.02%  kworker/u48:1  [kernel.kallsyms]     [k] cleanup_net
   - 94.74% cleanup_net
      - 94.64% ops_exit_list.isra.4
         - 41.61% default_device_exit_batch
            - 41.47% unregister_netdevice_many
               - rollback_registered_many
                  - 40.36% netdev_unregister_kobject
                     - 14.55% device_del
                        + 13.71% kobject_uevent
                     - 13.04% netdev_queue_update_kobjects
                        + 12.96% kobject_put
                     - 12.72% net_rx_queue_update_kobjects
                          kobject_put
                        - kobject_release
                           + 12.69% kobject_uevent
                  + 0.80% call_netdevice_notifiers_info
         + 19.57% nfsd_exit_net
         + 11.15% tcp_net_metrics_exit
         + 8.25% rpcsec_gss_exit_net

It's very critical to optimize the exit path for network namespaces,
because they are destroyed under net_mutex and many namespaces can be
destroyed for one iteration.

v2: use dev_set_uevent_suppress()

Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>