net/i40e: improve vector Tx performance
net/i40e: improve vector Tx performance

For i40e vector Tx path, if tx_offload is set as FAST_FREE_MBUF mode,
no mbuf fast free operations are executed. To fix this, add mbuf fast
free mode for vector Tx path.

Furthermore, for i40e vector Tx path, if implement FAST_FREE_MBUF mode,
it means per-queue all mbufs come from the same mempool and have
refcnt = 1. Thus we can use bulk free of the buffers when mbuf fast free
mode is enabled.

For vector path in arm platform:
In n1sdp, performance is improved by 18.4%;
In thunderx2, performance is improved by 23%.

For vector path in x86 platform:
No performance changes.

Suggested-by: Ruifeng Wang <a548d51fbc42485a8ba807711a537e88c2b59dd2@arm.com>
Signed-off-by: Feifei Wang <2493857810266eaae720c9869c8b9d6ce45d3739@arm.com>
Reviewed-by: Ruifeng Wang <a548d51fbc42485a8ba807711a537e88c2b59dd2@arm.com>

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