app/testpmd: set maximum LRO packet size

app/testpmd: set maximum LRO packet size

This patch implements use of the API for LRO aggregated packet
max size.
It adds command-line and runtime commands to configure this value,
and adds option to show the supported value.
Documentation is updated accordingly.

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>
Acked-by: Matan Azrad <matan@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>