app/testpmd: fix metadata API and Tx insertion

app/testpmd: fix metadata API and Tx insertion

Previous patch introduces the Tx metadata feature, with unnecessary
restrictions on data entry.
It also used the metadata in txonly fwd engine only.

This fix removes the data entry restrictions on metadata item.
It also implements callback function to add the metadata in every
Tx packet, sent by any fwd engine.

Fixes: c18feafa193c ("app/testpmd: support metadata as flow rule item")

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Ori Kam <orika@mellanox.com>