app/testpmd: create mbuf based on max supported segments

app/testpmd: create mbuf based on max supported segments

Configuring buffer size based following parameters:
- max-pkt-len
- max supported segments per MTU

Buffer size are configured as given below:
- If platform supports infinite segments per packet then default
  buffer size is used.
- If platform supports nb_mtu_seg_max segments then buffer size
  is configured as (max-pkt-len / nb_mtu_seg_max) + headroom

Signed-off-by: Sunil Kumar Kori <skori@marvell.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>