ovs: Remove rtnl_lock() from ovs_exit_net()

Here we iterate for_each_net() and removes
vport from alive net to the exiting net.

ovs_net::dps are protected by ovs_mutex(),
and the others, who change it (ovs_dp_cmd_new(),
__dp_destroy()) also take it.
The same with datapath::ports list.

So, we remove rtnl_lock() here.

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