app/testpmd: do not allow dynamic change of core number

app/testpmd: do not allow dynamic change of core number

When the number of forwarding cores changed in runtime, the issue may
be encountered:
If the nbcore set little than current nbcore, the forwarding thread
will still running on the extra cores. Therefore, trying to stop
forwarding will hang testpmd, since it will wait for the extra cores to
stop.

So do not allow to change nbcore number when forwarding is running.

Fixes: 0c0db76f42ed ("app/testpmd: separate forward config setup from display")
Cc: stable@dpdk.org
Signed-off-by: Zhenghua Zhou <zhenghuax.zhou@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>