app/testpmd: use per-core variable in flowgen
app/testpmd: use per-core variable in flowgen

Use per-core variable for flow indexing to solve cache contention in
multi-core scenarios.

Signed-off-by: Zhihong Wang <d340c26e7e7f43270f08407698de08b85b54ac43@bytedance.com>
Acked-by: Xiaoyun Li <151cc00bdba86cfa2ecfc39aaa98def8f092d9d8@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk