app/testpmd: support sending cloned packets in flowgen

app/testpmd: support sending cloned packets in flowgen

When testing high performance numbers, it is often that CPU performance
limits the max values device can reach (both in pps and in gbps)

Here instead of recreating each packet separately, we use clones counter
to resend the same mbuf to the line multiple times.

PMDs handle that transparently due to reference counting inside of mbuf.

Reaching max PPS on small packet sizes helps here:
Some data from our 2 port x 50G device. Using 2*6 tx queues, 64b packets,
PowerEdge R7525, AMD EPYC 7452:

./build/app/dpdk-testpmd -l 32-63  -- --forward-mode=flowgen \
  --rxq=6 --txq=6  --disable-crc-strip --burst=512 \
  --flowgen-clones=0 --txd=4096 --stats-period=1 --txpkts=64

Gives ~46MPPS TX output:

  Tx-pps:     22926849          Tx-bps:  11738590176
  Tx-pps:     23642629          Tx-bps:  12105024112

Setting flowgen-clones to 512 pushes TX almost to our device
physical limit (68MPPS) using same 2*6 queues(cores):

  Tx-pps:     34357556          Tx-bps:  17591073696
  Tx-pps:     34353211          Tx-bps:  17588802640

Doing similar measurements per core, I see one core can do
6.9MPPS (without clones) vs 11MPPS (with clones)

Verified on Marvell qede and atlantic PMDs.

Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>