examples/kni: fix crash while handling userspace request

examples/kni: fix crash while handling userspace request

When KNI interface receives RTE_KNI_REQ_CFG_NETWORK_IF request, it
stap/start the physical device which as a result of stop() can free all
the mbufs in its queue.
Meanwhile sample application continues to read from KNI interface queues
and push into device queues. This simultaneous access may cause a crash,
crash log can be found at defect description.

As a solution KNI sample application can do the proper synchronization,
and stop transfer between KNI interface and physical interface while
physical device stop/started.

Bugzilla ID: 116
Fixes: 3fc5ca2f6352 ("kni: initial import")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Phil Yang <phil.yang@arm.com>