examples/kni: fix crash during MTU set

examples/kni: fix crash during MTU set

During MTU set (kni_change_mtu) sample application setup queues, which
can free and re-allocate queues.
Meanwhile sample application keeps continues in Rx/Tx burst calls in
different threads, which may cause crash during queue setup.

Pausing application Rx/Tx calls before MTU set and starts it back
afterwards.

Bugzilla ID: 482
Fixes: a26b116749a3 ("examples/kni: fix MTU change to setup Tx queue")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Rasesh Mody <rmody@marvell.com>
Tested-by: Xi Zhang <xix.zhang@intel.com>