kni: allow per-net instances

kni: allow per-net instances

There is a global variable 'device_in_use' which is used to make sure
only one instance is using /dev/kni device. If you were using LXC, you
will find there is only one instance of KNI example could be run even
different namespaces were created.

In order to have /dev/kni used simultaneously in different namespaces,
making all of global variables as per network namespace variables.

With regard to single kernel thread mode, there will be one kernel
thread for each of network namespace.

Signed-off-by: Dex Chen <dex.chen@ruckuswireless.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>