ethdev: support metadata as flow rule criteria

ethdev: support metadata as flow rule criteria

As described in [1], a new rte_flow item is added to support metadata
to use as flow rule match pattern.
The metadata is an opaque item, fully controlled by the application.

The use of metadata is relevant for egress rules only.
It can be set in the flow rule using the RTE_FLOW_ITEM_META.

An additional member 'tx_metadata' is added in union with existing member
'hash' of struct 'rte_mbuf', located to avoid conflicts with existing
fields. This additional member is used to carry the metadata item.

Application should set the packet metadata in the mbuf dedicated field,
and set the PKT_TX_METADATA flag in the mbuf->ol_flags.
The NIC will use the packet metadata as match criteria for relevant
flow rules.

This patch introduces metadata item type for rte_flow RTE_FLOW_ITEM_META,
along with corresponding struct rte_flow_item_meta and ol_flag
PKT_TX_METADATA.

[1] "[RFC,v2] ethdev: support metadata as flow rule criteria"

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>